Change CMake logic for lib cdi_ndi to ensure upstream code can build when NDI is not found

[keadyk]

Background

• previously, the cdi_ndi library wasn't built at all when NDI was not found
• this is fine in theory, but it made things tricky in upstream code; because the NDI_FOUND cmake variable was not available, there was no obvious way to protect against includes of draco cdi_ndi header files that were not installed.

Purpose of Pull Request

• Instead of omitting the cdi_ndi lib entirely when NDI is not found, stub out the routines that need NDI (have them throw assertions)
• This means that upstream code can be built without protecting cdi_ndi includes in macros -- the calls to cdi_ndi will just throw if it is not found
• This is needed to get a corresponding pull request in gitlab going (I will update the description of that PR shortly)

Description of changes

• update CMakeLists logic for cdi_ndi - there was previously a confusing "if(NDI_FOUND) and not environment(travis)" statement wrapping the cmake logic. I think it was trying to protect against building the cdi_ndi lib in CI -- but you'd think that would be automatic because NDI_FOUND would be false... (?!)
• Move ndi.h include into config file, and remove from other cdi_ndi files
• Wrap actual calls to ndi routines / references to env. variables in NDI_FOUND macro
• @KineticTheory I am open to other ways to do this in CMake, and also more than willing to explain why I did things this way :)

Status

• Reference:
  • Pre-Merge Code Review
  • CDash Status
• Checks
  • Travis/Appveyor 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

[KineticTheory]

@keadyk I'll review this over the weekend.

[keadyk]

@KineticTheory Thank you -- no rush!! Hopefully my latest commit fixes the last few warnings :)

[codecov]

Codecov Report

Merging #818 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           develop    #818   +/-   ##
=======================================
  Coverage     94.2%   94.2%           
=======================================
  Files          367     367           
  Lines        17259   17259           
=======================================
  Hits         16263   16263           
  Misses         996     996           

[KineticTheory]

LGTM