Some cleanup to the pystencils runtime headers

src/pystencils/backend/platforms/cuda.py

src/pystencils/include/PyStencilsField.h

src/pystencils/include/pystencils_runtime/half.h

src/pystencils/include/pystencils_runtime/hip.h

src/pystencils/jit/gpu_cupy.py

src/pystencils/types/types.py