fix names of x,y coordinates fields of domcfg

As discussed at the bottom of MR !32 (merged), the files produced by DOMAINcfg assign the same name 'x' and 'y' to both dimensions and coordinates.

Adding the correct name to coordinates fields, namely nav_lat and nav_lon, will simplify the handling of these files, e.g. with python packages such as xarray (as confirmed by @malmansi).