Fix guards OpenMP and GPU execution policy guards in signed_distance API

[kennyweiss]

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 #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
  

  axom/src/axom/quest/interface/signed_distance.hpp

  Lines 87 to 93 in 9ea7c93

  	// Signed Distance execution policies
  	enum class SignedDistExec
  	{
  	CPU = 0,
  	OpenMP = 1,
  	GPU = 2
  	};

  
  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.

[cyrush]

since this could be an issue in multiple places, should we move to use axom-wide execution polices?
(instead of ones for specific algorithms)