I don't get it. What's so special about 0 (which implies the Airy 1830 ellipsoid) that it can be ignored, but other ellipsoids (e.g. 2 => GRS80) raise an error? It's not like the code actually creates/assumes an Airy ellipsoid.

In the light of which, it seems the implementation should say one of three things:
 1. *All* ellipsoids are unsupported and so should raise an error.
 2. Or, we just quietly ignore *all* ellipsoids.
 3. Or, we actually *support* some ellipsoids (i.e. they result in an appropriate CoordSystem) and raise errors or ignore the others.

As it stands, this PR takes a murky implementation and makes it murkier.