GCMC region methods fail on OpenCL and without a region #38

delete_waters() and num_waters() upload the GCMC sphere centre to the GPU twice. CUDA tolerates this, but OpenCL raises TypeError: a bytes-like object is required, not 'Array', so both methods fail there. Neither was covered on that platform: loch's OpenCL tests only exercise move().

Separately, delete_waters() raises AttributeError on _reference_indices when no GCMC region is defined, which is the same problem fixed for num_waters() in #36.