Optimize VCF reading time

[biona001]

In the documentation, it reads

For optimal performance, it is best to filter the VCF file down to records between start_bp and end_bp (e.g. with bcftools) before running solveblock.

We can automate this step internally within solve_block() (if the input VCF file is indexed) by calling bcftools using the bcftools_jll.jl package.