[python] Files re-work on io/ingest.py

[johnkerl]

The file apis/python/src/tiledbsoma/io/ingest.py is quite large.

• At very least split it up into io/ngest.py and io/outgest.py
• This may entail some common symbols moved to a new io/_common.py

Possible follow-on (I prefer as a separate PR from the above):

• io/ingest.py -> io/ingest_anndata.py (for from_anndata and from_h5ad)
• io/outgest.py -> io/outgest_anndata.py (for to_anndata and to_h5ad)
• io/ingest_whatevs.py and io/outgest_whatevs.py for new spatial file-format experiments

This is intended to be a warm-up issue for a new team member to be selected by @jp-dark.

Please also see
https://github.com/single-cell-data/TileDB-SOMA/wiki/Information-for-developers
and don't hesitate to contact me with any questions about TIleDB-SOMA development. :)

[johnkerl]

@ryan-williams I believe this is complete (unless you had a 2nd PR in mind) -- if I'm mistaken we can re-open -- thank you!

[ryan-williams]

Sounds good, I was thinking we can revisit the 2nd part (format-specific {ingest,outgest} files) later, if/when we support more formats…

[johnkerl]

Thanks -- I'll make a separate issue