Added a very simple update method that modifies the display_xref_id field and checks on whether the number of attached exons has increased.
Added a very simple update method that modifies the display_xref_id
field and checks on whether the number of attached exons has increased.

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