Added VDJ concatenation modules and added MuData object

[saraterzo]

Added VDJ concatenation module to concatenate "filtered_contig_annotation" files from scirpy package.
Added MuData module to create MuData objects to handle VDJ, and CITE-seq modalities. Specifically MuData object is built only for filtered count matrices (not raw) from GEX and CITE-seq modalities.

• This comment contains a description of changes (with reason).
• If you've fixed a bug or added code that should be tested, add tests!
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• Usage Documentation in docs/usage.md is updated.
• Output Documentation in docs/output.md is updated.
• CHANGELOG.md is updated.
• README.md is updated (including new tool citations and authors/contributors).