Based a Genotype class on VariantFieldValue

With a couple GT specific operators, including separating out the alleles.

Heavily based off htslib and comment stream in #77.
Ok... happy to oblige, but I'm going to need some C++ here, or a link to where I can copy this code from.

FYI, after going through several iterations (see previous daily commits), this *entire* class is just copied from https://github.com/broadinstitute/gamgee/blob/master/gamgee/variant_field_value.h with minor modifications and a code review from @MauricioCarneiro who said to delete the move / copy in this... well, copy. :smile: There are zero references to this `variant_deep_copy` function you mentioned in the original variant_field_value.h, so I'll need more info in how/when it should be used in this one-off.