Renamed `genes` object to more appropriate `genomicFeatures`

The `genes` object is intended to represent candidate genes or variants, making it a bit of a misnomer.

A number of alternatives were discussed in the Google doc:
* `variants`
* `variantGenes`
* `genetics`
* `sequenceFeatures`
* `genomicFeatures` (most popular)

I also fixed a potentially very-confusing typo in the description of the start and end positions (which are of the variant, not the gene). This is an uncorrected relic from before the first commit and is unfortunately incorrect in the v1 version.
The idea is that both causal genes (course-grained) and variants (fine-grained) genotypic data would be represented with this object. The reason that both levels are necessary is that variant-level genotypic information is not always known or shareable. Sometimes only a gene name can be provided.