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.
Hmm, I see what you mean, but I would argue that it's more complex since there aren't really two distinct options (gene or variant), it's a continuum. You might want to specify a gene along with a zygosity, and maybe a variant effect as well, without giving a position. It could probably be separated into the union of two types in Avro, one that has a chromosomal position and one that doesn't, though.