Accept loom file format as input

[sjfleming]

This will assist with a Broad DSP User Ed workshop in the near future, and could be helpful generally if people want to use the loom file format.

Since anndata is already a dependency, this is straightforward

[sjfleming]

It seems we need to introduce a dependency on loompy as well

[sjfleming]

Loom is also extremely flexible. We will have to obey some sort of spec about formatting conventions. I assume we will try to follow Optimus conventions

[sjfleming]

Here is the loom spec

https://broadinstitute.github.io/warp/docs/Pipelines/Optimus_Pipeline/Loom_schema/

[sjfleming]

Closed by #238