Creation of 2-d latitude and longitude coordinates #962

When 1-d projection x/y coordinates, or 1-d grid latitude/longitude coordinates exist with a coordinate reference, then we should be able to create the corresponding 2-d latitude/longitude coordinates on-the-fly if required (e.g. as is the case for regridding). It should also be possible to create theses 2-d coordinates permanently on a Field or Domain for future use.

Note, this is already possible to for HEALPix grids via the create_latlon_coordinates method, but we can extend this method to also cater for other spherical projects that imply 2-d lat/lon coordinates.

This will be fixed as part of #961, or which the functionality is required to cater to PP and UM datasets.