Casting options for nodata in Raster.from_array

Currently, the method only allows safe casting. This is a fine start, but this should eventually become more sophisticated. One option might be to add an explicit "casting" option, similar to the numpy.astype function.

Alternatively, we could consider using unsafe casting, but then issuing a warning when casting occurs. Would probably want an option to disable the warning - or elevate it to an error - if using this option.

I'm currently favoring adding both a "casting" option, and a "warn" option (which would issue a warning in the case of unsafe casting)