Add high-level Python API for VCF

[jeromekelleher]

After #349 is done we should be able to provide a stable and documented API for one-shot conversion of VCF. Let's get this done for the next release, as there's no current replacement for the sgkit APIs.

[jeromekelleher]

Note this is scheduled for the upcoming release over the next few days @pmoris - you should have a straightforward API available for all users then, given the bioconda package.

[pmoris]

That's great to hear! Nicely done and many thanks!

[jeromekelleher]

Closed in #406