Support VCF files as input

[mmyers1]

Current input file format is not commonly supported and requires custom treatment. Ideally, we would support using the common variant file format VCF. Potential routes:

1. Include a script to convert from VCF to CALDER-formatted input file.
2. Convert input/internal-use file format to VCF.

[immuneAI]

Hello @mmyers1,

Any update with this great step? It would be awesome if we can infer the HATCHET output and integrate into CALDER input.