SPHERICART_OPENMP and SPHERICART_ENABLE_CUDA are ignored when the compiler does not support them

[Luthaf]

If a user tries to build sphericart with either of these option set to ON, and cmake can not find the requirements, we will just print a message Could not find a CUDA compiler or Could not find OpenMP, and continue building the code as if these options where disabled.

I see how this is the most sensible when building the code from setup.py, where we want to enable these if possible but still build in the other case, but it might create issues for C++ users who think they built everything right but then get slowdown/crash later at runtime.

At the very least, both should be a warning when enabled but not found (currently only OpenMP is); and I would appreciate if we can turn these into errors without breaking the python build. Maybe we could have a IMPLICIT_DISABLE_IS_ERROR option, ON by default and set to OFF by setup.py.