Support for gVCF

[lijiayong]

Does vgraph compare gVCFs of various forms? Aside from the standard form on page 24 of gVCF spec where a symbolic alt allele <*> indicates a hom ref block, GATK uses a different symbolic alt allele < NON_REF >. Some gVCFs mark no call blocks rather than hom ref blocks, and some mark both. Does vgraph accept all these forms of gVCFs? Thanks a lot.