Add Variant Annotations to a VDS based on a KeyTable

The new annotate_variants_keytable command adds variant annotations based on a KeyTable.

For example, we can annotate variants by the pLI of the gene in which they lie:

pLI_per_gene = hc.import_keytable("pLI-per-gene.tsv", ['gene'])
pLI_annotated_vds ="my.vds")
  .annotate_variants_keytable(pLI_per_gene, 'va.pLI = table.pLI', ['va.gene']])

The third argument is an array of hail expressions whose types should match the keys of the key table. This expression sets va.pLI, for every variant, to the value of the pLI column from the row whose key matches the given variant’s gene.

I should stress that the array is an array of hail expressions, so this is also valid:"my.vds")
  .annotate_variants_keytable(a_key_table, ' =', ['exp( + va.gq_offset)']])