Support bgzipped vcf files

Background

We currently only support unzipped vcfs, however, not much work is required to support bgzipped vcfs. This is very useful in particular for testdata (and in extension, #2130 (closed)), where unzipped vcfs take up unnecessary much storage.

Implementation

Add support for bgzipped vcf in analysis_config.py. cyvcf2 should support bgzipped vcfs out of the box.