MAF/HWE for dosage data/a subset of individuals

[chris-german]

It would be a nice feature to allow for gtstats() to report MAF/HWE p-values for the dosage data in addition to the existing method for genotype data. Some GWAS pipelines use MAF in order to skip analysis for SNPs that have 0 MAF information. The genotype data can report 0 MAF (0 variation) when there is variation in the dosage data.

It would also be nice to be able to calculate these stats on a subset of the individuals in the VCF file by supplying a row (individual) mask.