get_embedding should not be obs biased

[ebezzi]

The get_embedding function was designed for obs embedding only (docs, parameters), but it also works for var embeddings. Changes needed:

1. Rename obs_soma_joinids to soma_joinids. This is a breaking change, so consider whether we want to rename (and break) or do this via deprecation
2. Change the docs to remove the obs bias.

[pablo-gar]

Superseded by the embedding arguments of get_anddata