support ellipsoids in healpix

[keewis]

This adds support for ellipsoids to the healpix grid object (healpix-geo is the only builtin DGGS implementation that supports it, h3ronpy would need to support the conversion to be efficient).

There's still a few things to do:

• tests
• support serializing / deserializing custom ellipsoids / spheres
• support custom sphere objects (the geodesy crate does not support inverse_flattening = 0.0 at the moment)

[keewis]

I can confirm that the small offset you'd get when plotting high-res data does not appear with this.

[keewis]

We'll wait on EOPF-DGGS/healpix-geo#94, which should make passing ellipsoid definitions much easier.