minor updates
minor updates

make referenceSequenceSet optional in ReadGroup for if there are no alignments
allow ncbi_taxon_id in ReferenceSequence to override the value in ReferenceSequenceSet
other comment changes
laserson/schemas,hjellinek/jeltje-schemas-sphinx,adamnovak/schemas,heuermh/ga4gh-schemas,buske/schemas,lh3/schemas,nlwashington/schemas,delagoya/schemas,delagoya/schemas,benedictpaten/schemas,Jeltje/ga4gh-schemas,sarahhunt/schemas,dnanexus-rnd/schemas,buske/schemas,benedictpaten/schemas,hjellinek/schemas,ga4gh/ga4gh-schemas,diekhans/ga4gh-schemas,nlwashington/schemas,dnanexus-rnd/schemas,hjellinek/schemas,ga4gh/schemas,kellrott/schemas,adamnovak/schemas,kellrott/schemas,dglazer/schemas,sarahhunt/schemas,dcolligan/schemas,Jeltje/ga4gh-schemas,diekhans/ga4gh-schemas,pcingola/schemas,laserson/schemas,saupchurch/schemas,macieksmuga/schemas,lh3/schemas