Assign coordinates in HRVSatellite (and probably Satellite and OpticalFlow and maybe others)

[JackKelly]

Here's what an on-disk HRVSatellite batch looks like now (from v15):

We should assign coordinates like this:

def _set_sat_coords(dataset: xr.Dataset) -> xr.Dataset:
    return dataset.set_coords(
        ["time", "channels", "y_geostationary_index", "x_geostationary_index", "y_osgb", "x_osgb"]
    )

Which results in this:

[JackKelly]

This is a sub-task of #629