* update "nitro"

* update coda-oss and fix compiler errors

* update coda-oss and nitro with less use of sys::

* unit-test paths are wrong for "externals"

* update "nitro" to get SegmentMemorySource overloads

* latest from "nitro"

* match nitro -> nitf change

* make_span()

* latest from "coda-oss"

* more filesystem from coda-oss

* latest from "nitro"

* std::endian

* latest from "nitro"

* latest from "nitro" and "coda-oss"

* put common #includes for PCH in a single file

* build check_valid_six

* use EXIT_* rather than 0/1

* getting rid of std::auto_ptr<> because it's gone in C++17

* build all projects with C++17

* use <filesystem> to remove hard-coded paths

* in Visual Studio, run check_valid_six with a pre-defined path

* tweak plugin dir

* can't have "check_valid_six" as a directory for *ix build

* build crop_sicd

* turn on /Wall, fix some warnings

* C++17 generates more warnings that I don't want to fix right now

* removed some compiler warnings from third-party code

* comment-out use of C++17's <filesystem>

* tweak project settings for consistency

* unit-test for createFakeComplexData()

* turn on compiler warnings

* crank-up warning level

* enable "Warnings as errors"

* turn on all warnings (many #pragma'd away)

* get rid of more warnings

* adjust more #pragma's

* update "nitro"

* update coda-oss and fix compiler errors

* update "nitro"

* update coda-oss and fix compiler errors

* use "externals" from top-level "install"

* use int64_t, etc. instead of sys::Int64_T

* mem::SharedPtr -> std::shared_ptr

* update coda-oss and nitro with less use of sys::

* unit-test paths are wrong for "externals"

* update "nitro" to get SegmentMemorySource overloads

* use std::unique_ptr<[]> instead of mem::ScopedArray

* use int32_t et. al. instead of sys::Int32_T

* use std::byte (not quite working yet, but need to move code to a faster machine)

* build with std::byte

* build with simplified "externals" scheme

* latest from "nitro"

* match nitro -> nitf change

* make_span()

* use std::thread::hardware_concurrency() instead of sys::OS().getNumCPUs()

* sys::Off_T -> ptrdiff_t

* fix compiler errors from previous merge; mostly #include <thread>

* Revert "sys::Off_T -> ptrdiff_t"

This reverts commit aa22a37.

* sys::Off_T -> int64_t; sys::SSize_T -> ptrdiff_t

* latest from "coda-oss"

* use filesystem::path and std::endian

* more filesystem from coda-oss

* less use of sys::

* NULL -> nullptr

* use .data() rather than &d[0]

* mem::BufferView -> std::span

* const std::span<>& doesn't make much sense

* need make_span() overload for pointer+size

* latest from "nitro"

* std::endian

* coda-oss now gives us std::endian

* sys::Filesystem -> std::filesystem

* don't need to explicitly call fs::path::string() in most cases

* sys::Filesystem -> std::filesystem

* fix build errors when using C++17

* simplify access to some ImageSubheader values

* newReadControl() returns a std::unique_ptr<> rather than a raw pointer

* a singleton is much easier in C++11

* don't need to do manual locking in C++11

* use a template and some macros to dramatically simplify Enums

* adjust macros for GCC, provide operators for SWIG

* got the map entries wrong

* make getIndices() a bit more robust

* store a std::unique_ptr<> instead of a raw poniter

* throw exceptions in getIndices() rather than assuming

* use Enum<T>.toString() where possible rather than duplicating code

* remove duplicate code converting from a string to Enum<T>

* don't explicilty check for "SIDD" as that's already part of getDataType()

* use std::string instead of char[]

* reduce use of "delete[]"

* less use of "SICD" and "SIDD", use six::DataType instead

* simplify Region buffer management

* reduce some explicit use of std::shared_ptr<> with std::make_shared and/or auto

* more reduction in explicit use of std::shared_ptr

* latest from "nitro"

* simplify use of six::Region

* reduce use of explicit .toString()

* further reduction in explicit use of toString()

* template specialiaton can't be inline with G++

* simplify cphd enum creation

* reduce use of toString() with setAttribute()

* latest from "nitro" and "coda-oss"

* don't need catch(excep::Exception) ex.toString() as std::exception::what() now does the same thing

* really only have two calls to setAttribute(): std::string and size_t

* use macros to generate SIDD Enums

* add operator<< for Enum<T> to avoid some explicit .toString() calls

* add operator==(std::string) overload to redue need for .toString()

* add operator<<() to redue need for .toString()

* don't implmeent a bunch of six::toString() routines; they sholdn't be called anymore

* further reduction in use of toString()

* eliminate six::toString() template; just provide a few overloads instead

* createSixString() which calls six::toString(t) instead of t.toString()

* createSixString() overload with URI parameter

* allow different strings (e.g., "1" and "+1") to map to the same value

* put "Enum" support code in a separate file

* there is now just one NOT_SET_VALUE

* update "nitro"

* update coda-oss and fix compiler errors

* update coda-oss and nitro with less use of sys::

* unit-test paths are wrong for "externals"

* update "nitro" to get SegmentMemorySource overloads

* latest from "nitro"

* match nitro -> nitf change

* make_span()

* latest from "coda-oss"

* more filesystem from coda-oss

* latest from "nitro"

* std::endian

* latest from "nitro"

* latest from "nitro" and "coda-oss"

* latest from coda-oss

* lost toString<nullptr>()

* latest from coda-oss/main

* want "std" augmented in this repo

* update "nitro"

* update coda-oss and fix compiler errors

* update coda-oss and nitro with less use of sys::

* unit-test paths are wrong for "externals"

* update "nitro" to get SegmentMemorySource overloads

* latest from "nitro"

* match nitro -> nitf change

* make_span()

* latest from "coda-oss"

* more filesystem from coda-oss

* latest from "nitro"

* std::endian

* latest from "nitro"

* latest from "nitro" and "coda-oss"

* latest from coda-oss

* lost toString<nullptr>()

* latest from coda-oss/main

* want "std" augmented in this repo

* partial update from coda-oss/main

* latest from coda-oss/main

This reverts commit 0b99300.

This reverts commit e89da71.

…ibrary into develop/update_externals

This reverts commit da30b96.

This reverts commit e69391b.

This reverts commit c10f233.

…needed

…ibrary into develop/update_externals