Add support for alevin backend

[mfansler]

Users may prefer to use Alevin as the quantification backend as it offers an alternative deduplication model. Multiple tasks are necessary to establish this:

• add a new configuration key, e.g., backend that accepts either: "kallisto-bustools" or "alevin"
• create UTRome indices (mouse, human) for alevin, possibly with genomic decoy
• implement downloading rules (probably should do this as part of snakemake all the things #51)
• implement quantification rules
• implement output rules

[amv2672]

Do you have the human UTRome index created for alevin?

[mfansler]

No, this has not been created yet.