Protect TNReaction constructors in NDI_FOUND to suppress unreachable …

[keadyk]

…code warning in MSVC

Background

• PR 818 caused a build warning for MSVC in release mode; this attempts to fix it.

Purpose of Pull Request

• See above -- fixing a build warning in MSVC that I introduced in my last PR

Description of changes

• Move TNReaction constructors inside NDI_FOUND macro; if !NDI_FOUND, constructors are now stubs, and the base class (NDI_Base) will throw.
• Remove cdi_ndi/config include in derived TNReaction class; it is available through the base class NDI_Base
• only declare load_ndi function when NDI_FOUND

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

[codecov]

Codecov Report

Merging #819 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           develop    #819   +/-   ##
=======================================
  Coverage     94.2%   94.2%           
=======================================
  Files          367     367           
  Lines        17259   17259           
=======================================
  Hits         16263   16263           
  Misses         996     996           

[KineticTheory]

LGTM.