rtxpy hillshade does not handle np.nan values

[ianthomas23]

GPU code seems to hang if the supplied cupy-backed xarray.DataArray contains np.nan values. Need to check rtxpy viewshed for the same problem.

Possible solutions:

1. Keep the np.nan values (and np.inf too?) and deal with them gracefully.
2. Check for existence of non-finite values and raise an exception.
3. Document the limitation.