Deprecate GeomWithData .inner() method in favour of Deref

[urschrei]

This matches the CachedEnvelope impl

• [x] I agree to follow the project's code of conduct.

• [x] I added an entry to rstar/CHANGELOG.md if knowledge of this change could be valuable to users.

[adamreichold]

Hhhmmm, I am admittedly not completely sure about this as GeomWithData is as much Geom as it is Data, so I don't think it is obvious that it should dereference into the geometry instead of the data part. Not sure whether this is really similar to the CachedEnvelope case where nothing is added conceptionally.

[urschrei]

Hmm good point. Let's abandon this for now…