Allow geometry selection independent of VecGeom being enabled

[sethrj]

This does something similar with geometry that we have for the RNGs: selecting in-house geometry while still building the others. I'm thinking about adding a "simple ORANGE" for performance testing, and it'll be good to have it available without a branch.

This replaces a lot of #if CELERITAS_USE_VECGEOM with #if CELERITAS_GEO == CELERITAS_GEO_VECGEOM, which I think makes it a little clearer that it's about the main geometry selection rather than the availability of the external library.

I also renamed load_gdml to make it clearer that it's loading into the global geant4 state as opposed to VecGeom. The way it also tried to handle memory was also wrong: the GDML reader isn't actually returning a new'd pointer with C++ style memory management; the underlying object saves this to a magical vector that manages memory.

I also reworked the VecGeom test (since it can now be enabled without VecGeom being selected as the "core" geometry) so that it can run all the Geant4 geometry tests without having to break them apart into separate test instances or loading geant physics.

[sethrj]

@pcanal Any further comments? I think I've addressed all your concerns. If you want a clearer error message perhaps the following is good enough:

diff --git a/cmake/CeleritasUtils.cmake b/cmake/CeleritasUtils.cmake
index b70f8abc..77d3d304 100644
--- a/cmake/CeleritasUtils.cmake
+++ b/cmake/CeleritasUtils.cmake
@@ -298,8 +298,9 @@ function(celeritas_define_options var doc)

   list(FIND ${var}_OPTIONS "${${var}}" _index)
   if(_index EQUAL -1)
+    string(JOIN "," _optlist ${${var}_OPTIONS})
     celeritas_error_incompatible_option(
-      "Invalid selection not in list: \"${${var}_OPTIONS}\""
+      "Invalid option for ${var}: must be in the list [${_optlist}] "
       "${var}" "${_default}"
     )
   endif()

[pcanal]

Thanks