Fixed units bug that prevents regridding with no coord_system

With the switch to a single implementation for linear regridding we introduced a bug that can prevent regridding of a cube with coords that have no coord system. This was partially supported, but limited to cases where the units of the coords were metres. Cubes with a horizontal grid with coord_systems of `None` are common (e.g. when loading from netcdf on a regular lat-lon) and are often in degrees causing the code to throw an exception. This PR fixes the problem allow regridding provided the coord metadata matches i.e. 'latitude' -> 'latitude', degrees -> degrees and so on.
@esc24 I know that you're re-factoring the previous code, but what if the units are `km` or `miles` ... or some other distance unit derivative. Is that possible or make sense? Do we need to cover that?