Previously OpenMP was necessary to build the project. Missing OpenMP
should no longer fail the build. This can now be configured with CMAKE
flags analog to CUDA.
RAYX_ENABLE_OPENMP builds with OpenMP support if available (Default ON).
RAYX_REQUIRE_OPENMP makes it required (Default: OFF).
Rayx should already fall back to single threaded sequential execution if
the corresponding Alpaka flags for faster devices are disabled. (see
Platform.h)

Callbacks work

WIP