Fix guards OpenMP and GPU execution policy guards in signed_distance API

A user reported an error in how we handle execution policy guards in the signed distance API.
Specifically, we're using #ifdef in some cases but should actually be using #ifndef guards.

See https://github.com/LLNL/axom/pull/1191 for a proposed bugfix.
(Specifically, the changes to signed_distance.cpp)

We might consider using this as an opportunity to clean up this section of the code.
Specifically, should the SignedDistExec::OpenMP enum case be defined in configurations that don't have OpenMP? Or would it be better to avoid this problem by either

• defining only the valid enum cases for the configuration
  E.g. something like changing the enum definition from this
  https://github.com/LLNL/axom/blob/9ea7c93c4b802b4da444727b995e4f02df344b3a/src/axom/quest/interface/signed_distance.hpp#L87-L93
  to

enum class SignedDistExec
{
  CPU = 0
#if defined(AXOM_USE_OPENMP) && defined(AXOM_USE_RAJA)
  , OpenMP = 1
#endif
#if defined(AXOM_USE_GPU) && defined(AXOM_USE_RAJA)
  , GPU = 2
#endif
};

• or, keeping the enum as it is and adding an isValid(SignedDistanceExec) method that can better isolate the checks

This might depend on expectations about how users are using the SignedDistanceExec enum.

Start with the proposed changes to src/axom/quest/interface/signed_distance.cpp in pull request #1191, then read the SignedDistExec definition in src/axom/quest/interface/signed_distance.hpp. Review the OpenMP and GPU configuration guards and determine whether enum cases should remain available in unsupported configurations or be conditionally defined. Done means the signed-distance API has correct guards and a decided, consistent handling of invalid execution policies.