[rare variant] Add support for STAAR

[akotlar]

Background:

A concise description of why we're pursuing this task.

From Thomas:

I would seem like a very low hanging fruit for bystro would be to make running something like this - https://github.com/xihaoli/STAAR - for rare variant analysis easier. No need to parse a large file, etc. It seems like a reasonable approach to doing rare variant analysis – at least as good as any I’ve seen. I’d be keen to give it a whirl with ADSP’s data.

Completion Criteria:

What is the goal, and how do we know this task is accomplished?

Create a STAAR port in Python. Initial version does not need to be scalable past a single node

Implementation Summary

Give enough technical detail for a teammate to understand the proposed solution.

Implement the R package under https://github.com/bystrogenomics/bystro/tree/master/python/python/bystro/rare_variant