CarpetX: avoid features not supported by nvhpc's compiler

[rhaas80]

This avoids two features not supported by nvhpc version nvc++ 24.3 ("nvc++ 24.3-0 linuxarm64 target on aarch64 Linux -tp neoverse-v2"):

• does not support custom OpenMP reductions
• RANK() is not a F90 function but a GNU extension

The former is the more annoying fix since I added an #ifdef. If one consider the possible loss in speed due to using a #pragma omp critical instead of (possibly) are more optimal implementation via #pragma reduction(...) negligible then one can remove the #ifdef.

[lwJi]

I confirm the this change make CarpetX compile on vista with nvc compilers

[rhaas80]

Alright, I updated the C++ stuff with auto &foo = bar and ran through clang-format-19 which (predictably) indented lots of code :-(

[rhaas80]

@lwJi please click on "approve" if you are happy with the proposed change.

[lwJi]

@lwJi please click on "approve" if you are happy with the proposed change.

I can't find a "approve" button on my side

[rhaas80]

@eschnett are you happy with the changes I had implemented? This would still be good to merge in.