Intermediate push with GFV back as subclass of VFV<int32_t>. This doesn't compile as still figuring out how writing to consts actually works in current master VFV.
https://github.com/broadinstitute/gamgee/blob/d077c4ee9704d5488f3381142493a28078dfa1d0/gamgee/variant_field_value.h#L99

Also figuring out overloading of move and other operators.
Biggest TODO: use GT instead of PL from htslib.