add strand option to allele_string; simplify code in ambig_code to use it
add strand option to allele_string; simplify code in ambig_code to use it

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