Clean up callSampleId and Name fields

- make callSampleId on GACall optional
- remove callSampleIds and callSampleNames from GAVariantSet
- add a comment clarifying that if callSampleId is not on a GACall, the results from a /callsamples/search?variantSetIds=x will be returned in the same order as the calls on a variant in that set
(and so the ith result maps to the ith call)

This means we can support both sparse and dense representations without too many differences in the API.
I'm not sure if this is a weird artifact of Github's web viewer or intentional for formatting; are these supposed to be backticks?