extract_tracts.py outputs gzipped vcfs instead of bgzipped

[pettyalex]

Hello,

To work with most VCF tooling, VCFs should be bgzipped. Python scripts can easily output bgzipped VCFs using pysam: https://pysam.readthedocs.io/en/latest/api.html .

If the VCF writing code were reworked to use pysam.VariantFile, it could natively handle both bgzipped and uncompressed files. https://pysam.readthedocs.io/en/latest/usage.html#working-with-vcf-bcf-formatted-files

This would substantially improve the usability of tractor on larger datasets.