Read pedigree data from VCF

[timothymillar]

VCF 4.3 allows pedigree data to be stored in a VCF header. This could be translated to the parent_id variable.

I haven't actually come across this "in the wild" but it's something I'd like to look into.

Related to #1014

[tomwhite]

Good idea - I haven't really looked at the pedigree stuff in VCF before.