BaseParser: get_direct_xref() can now return all matching IDs
BaseParser: get_direct_xref() can now return all matching IDs

It is possible for a (stable_id, linkage_xref) pair to match multiple direct
xrefs, for example when a MIM xref mapped to an Ensembl gene has been inserted
into both MIM_GENE and MIM_MORBID. Extend get_direct_xref() so that it returns
a list of all matching xref_ids when called in list context.

For backwards compatibility, get_direct_xref() still returns the first xref_id
found when called in scalar context. That said, it would appear that no parsers
in the tree actually USE this method now so we might consider simply removing
this bit of code.

james-monkeyshines/ensembl,Ensembl/ensembl,Ensembl/ensembl,james-monkeyshines/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,Ensembl/ensembl,james-monkeyshines/ensembl