Make outputs CF-compliant

Currently the model outputs (after converting to xarray) have lined up coordinates and the correct units, but for downstream evaluation and comparison with other models it would be nice to have our outputs 'CF compliant'. I think this is mostly a case of ensuring we use the right coordinate names with appropriate units, and including a standard_name where available.

This might be most easily achieved by converting the xarray object to an Iris Cube, for which there is already an easy conversion method in xarray, and internally knows about cf-standards.