Sort out formatting of add_Exon() and use assert_ref() too, before fixing problem reported by Mark McDowall.
Sort out formatting of add_Exon() and use assert_ref() too, before
fixing problem reported by Mark McDowall.

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