added ambig_code to calculate the ambiguity code from the allele_string
added ambig_code to calculate the ambiguity code from the allele_string

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