Added parameter checking to add_to_xrefs method
Added parameter checking to add_to_xrefs method

muffato/ensembl,mjg17/ensembl,at7/ensembl,at7/ensembl,mjg17/ensembl,Ensembl/ensembl,at7/ensembl,willmclaren/ensembl,at7/ensembl,willmclaren/ensembl,Ensembl/ensembl,danstaines/ensembl,james-monkeyshines/ensembl,muffato/ensembl,danstaines/ensembl,mjg17/ensembl,muffato/ensembl,mjg17/ensembl,Ensembl/ensembl,james-monkeyshines/ensembl,james-monkeyshines/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,danstaines/ensembl,muffato/ensembl