Add function to create genotype calls from genotype probabilities

[eric-czech]

For bgen data, we need a function that can produce hard calls from probabilities. This is necessary for calculating HWE in the UKB QC workflow (https://github.com/pystatgen/sgkit/issues/67). The function would look like (variants, samples, genotypes) float -> (variants, samples, ploidy) int8 and would only need to work for biallelic, diploid datasets for now.

[eric-czech]

For reference:

• https://glow.readthedocs.io/en/latest/api-docs/pyspark-functions.html?highlight=hard#glow.functions.hard_calls
  • source code: https://github.com/projectglow/glow/blob/f6476a924153b02fa29634580f1b467a4d48b714/core/src/main/scala/io/projectglow/sql/expressions/VariantUtilExprs.scala#L286
• https://hail.is/docs/0.2/methods/impex.html#hail.methods.import_bgen