resolve issue #20 per krttrr's suggestion
resolve issue #20 per krttrr's suggestion

The expected behavior is to only use alternate genotypes.

ashishjain1988/vcflib,vcflib/vcflib,jewmanchue/vcflib,glennhickey/vcflib,zhmz90/vcflib,travc/vcflib,andersje/vcflib,harshinamdar/vcflib,glennhickey/vcflib,harshinamdar/vcflib,travc/vcflib,andersje/vcflib,harshinamdar/vcflib,wzugang/vcflib,jewmanchue/vcflib,zeeev/vcflib,wzugang/vcflib,ashishjain1988/vcflib,wzugang/vcflib,ashishjain1988/vcflib,ashishjain1988/vcflib,glennhickey/vcflib,hyphaltip/vcflib,glennhickey/vcflib,zhmz90/vcflib,ashishjain1988/vcflib,andersje/vcflib,vcflib/vcflib,zhmz90/vcflib,harshinamdar/vcflib,hyphaltip/vcflib,tomdcsmith/vcflib,wzugang/vcflib,ashishjain1988/vcflib,jewmanchue/vcflib,ekg/vcflib,vcflib/vcflib,zeeev/vcflib,zeeev/vcflib,hyphaltip/vcflib,zeeev/vcflib,harshinamdar/vcflib,vcflib/vcflib,hyphaltip/vcflib,andersje/vcflib,zhmz90/vcflib,ekg/vcflib,hyphaltip/vcflib,glennhickey/vcflib,harshinamdar/vcflib,wzugang/vcflib,wzugang/vcflib,vcflib/vcflib,hyphaltip/vcflib,jewmanchue/vcflib,ekg/vcflib,tomdcsmith/vcflib,vcflib/vcflib,zhmz90/vcflib,jewmanchue/vcflib,glennhickey/vcflib,tomdcsmith/vcflib,andersje/vcflib,zeeev/vcflib,andersje/vcflib,ekg/vcflib,travc/vcflib,vcflib/vcflib,ekg/vcflib,zeeev/vcflib,jewmanchue/vcflib,vcflib/vcflib,zhmz90/vcflib