Implement self[item] = value for SkyCoord

The right hand value must be strictly consistent with self: - Identical class - Equivalent frames - Identical representation_types - Identical representation differentials keys - Identical frame attributes - Identical “extra” frame attributes (e.g. obstime for an ICRS coord)

With these caveats the setitem ends up as effectively a setitem on the representation data.

self.frame.data[item] = value.frame.data