added method derived allele (thanks to Michael) to return the derived allele (if defined ancestral allele)
added method derived allele (thanks to Michael) to return the derived allele (if defined ancestral allele)

willmclaren/ensembl-variation,willmclaren/ensembl-variation,Ensembl/ensembl-variation,willmclaren/ensembl-variation,willmclaren/ensembl-variation,dbolser-ebi/ensembl-variation,Ensembl/ensembl-variation,Ensembl/ensembl-variation,Ensembl/ensembl-variation,dbolser/ensembl-variation,dbolser-ebi/ensembl-variation,dbolser-ebi/ensembl-variation,dbolser/ensembl-variation,dbolser/ensembl-variation,Ensembl/ensembl-variation,dbolser/ensembl-variation,willmclaren/ensembl-variation