Add unit tests for loader functions

[stefsmeets]

I use this little snippet for testing the data in the __init__.py file. It would be a good idea to build some basic unit tests around it.

    VERBOSE = True

    for mip_table in (
            'Amon',
            'day',
    ):
        print()
        print(f'Loading `{mip_table}`')
        ts = load_timeseries_cubes(mip_table)

        first_cube = ts[0]
        for i, cube in enumerate(ts):
            print(i)
            cube.regrid(grid=first_cube, scheme=iris.analysis.Linear())