Adds the summarize_genotypes command described in #172 (closes #172). This command gives basic stats on a collection of ADAMGenotype records. Also adds methods to compute the type of a variant (SNV, insertion, deletion, etc.) in RichADAMVariant, which closes #174, and cleans up the no longer used VariantType enum in adam.avdl.
Merge pull request #187 from hammerlab/variant-stats-squashed

Add summarize_genotypes command