Convert various single-cell formats to zarr

[ryan-williams]

Tracking some data-format-conversion needs/functionality here.

WIP script for some conversions is at cli.py

Format	Read			Write to Zarr
	Locally	From cloud storage	In parallel w/ Spark	Locally	To cloud storage	In parallel w/ Spark
10x (.h5)	✅			✅	✅
AnnData (.h5ad)	✅			✅	✅
Loom (.loom)

Notes:

• AnnData's read_loom method loads the whole dataset into memory, which we'd like to avoid
• h5py only reads from local files
  • adding support there for reading from cloud-storage would be nice, but seems pretty nontrivial
  • @tomwhite has an implementation of this in java that we might rather rely on