Return genotype probabilities and dosages

[eric-czech]

We may also want to consider some of the Hail 0.1 features (import_bgen) that disregard calls where sum of genotype probabilities is over some threshold distance from 1. It also normalizes probabilities to sum to 1 -- though I'm confused on how that works if bgen stores all but 1 genotype probability. These features may only apply to version 1 of the bgen spec.