MuData conversion

[grst]

Description of feature

When processing multimodal data (e.g. CITEseq via cellranger multi, RNA+ATAC via cellranger-arc), we should not only generate a single AnnData/h5ad object, but instead combine the modalities into a MuData/h5mu object.

[saraterzo]

I have added in the pipeline a module to handle multimodal data using MuData object, specifically with CITE and VDJ so far.
@heylf, are you still working on this? Otherwise, I could start a draft PR.

[heylf]

Removed myself you can start a draft PR

[saraterzo]

Hi @heylf @grst
I have completed the PR #435.
Now it is ready for review :)