bugfix in vcf-filter-gt:

Equivalent reference alleles with different representations weren't
always being matched up. Now, reference alleles are represented as
pos=0, ref="", alt="".
unhide vcf-compare-gt command
ignore hom ref calls in vcf-compare-gt
add SampleData::removeFilteredWhitelist

removes filtered genotypes except those filtered by filters named in
the given whitelist
add vcf-remove-filtered-gt command

null out filtered genotypes (with optional filter whitelist)