Add support for BCF format

[cmdoret]

MODO now supports remote streaming of CRAM files. The test dataset of the genome center is mainly BCF files, therefore we need to implement the same feature for variant files.

Requirements:

• [x] Make sure we can BCF files
• [ ] metadata are stored in /data
• [x] files can be read locally
• [x] files can streamed
• [x] output accessible as a pysam.VariantRecord

[AssafSternberg]

stream_genomics already supports BCF files, both reading and writing to,

• for remote files BCF can be sliced and streamed or saved locally as a BCF file.

• local files BCF files can be read and streamed or saved as BCF or VCF (.vcf or .vcf.gz) and VCF files can be sliced and saved as BCF files.