Feature add atomics

[timmah]

Background

• Add atomic support for floating point types. This will be obsoleted when Draco adopts C++20.

Purpose of Pull Request

• add support for atomic fetch-and-add for floating point types
• it works, can't say it's great performance

Description of changes

• Add ds++/atomics.hh (must...not...call it house_atomics.hh)
• Add multithreaded unit test in ds++/test/tstatomics.cc that attempts to demonstrate correct use of atomics
• Requires C++11 support

Status

• Reference:
  • Pre-Merge Code Review
  • CDash Status
• Checks
  • Travis CI checks pass
  • Code coverage does not decrease
  • Valgrind test passes
  • Toss3 checks pass
  • Trinitite checks pass
  • Code reviewed/approved, sufficient DbC checks, testing, documentation

[codecov]

Codecov Report

Merging #592 into develop will increase coverage by <.1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           develop    #592     +/-   ##
=========================================
+ Coverage     93.7%   93.7%   +<.1%     
=========================================
  Files          377     381      +4     
  Lines        17665   17805    +140     
=========================================
+ Hits         16554   16687    +133     
- Misses        1111    1118      +7

[KineticTheory]

@timmah This looks really good. I have a couple of nit-picky changes that I want to see added before it is merged.

@atom @alexrlongne Can either of you review this PR?

[alexrlongne]

@KineticTheory I agree with your suggestions, otherwise this looks good to me! I think it's great that the standard has this level of control for atomics. I don't quite understand why they don't provide these kinds of basic operations on atomic types.

[timmah]

I want to add a test of fetch_sub, maybe hold off before merging.

OK--added a test of fetch_sub (which revealed a mistake--woot)

[attom]

This looks reasonable to me. I assume there's no problem using compare_exchange_weak instead of strong?