Support for VCFs without sample info

[brendanofallon]

In some cases it's handy to be able to compare VCFs without sample information - for instance, to see which variants in some query VCF match those in, say, the 1000G or ExAC database VCFs. As a short-term kludge I think this could be done by creating a fake sample for VCFs without samples, and having the variants all modeled as homozygous.