Grid interpolation does not maintain all coordinates in orginal datasets

[brogalla]

nemo_python/interpolation.py

Line 612 in f380fa2

A_W = 0.5*(A.isel(x=0)+A.isel(x=-1))

When halo=False, this line wipes out some of the pre-existing coordinates in the xarray dataset (e.g. nav_lon, nav_lat) and the next concatenation line then throws the error:
ValueError: Variables {'nav_lat', 'nav_lon'} are coordinates in some datasets but not others.

I've rewritten this code snippet to fix it in commit fa40a8b. In my test case it created the same results, but @knaughten please check to make sure it didn't break your code/works for you.