added ReferenceMapping for Genotype, filterByOverlappingRegion for GenotypeRDDFunctions

> If so, then you need to make it nullable. If not, then we don't need to worry about the null check here as you say.

Since it isn't nullable now, no reason to do the check.

> Would you ever want to project a Genotype without the variant?

Let's discuss this on the [PR](https://github.com/bigdatagenomics/bdg-formats/pull/39/files) that makes the change. Conceivably, you _could_, but not sure if it makes sense to allow people to.