Changes for xref for release 114

[TamaraNaboulsi]

Description

During the 113 xref run, several issues were flagged and these changes attempt to fix them. This is a sister PR to ensembl/714

Use case

These changes consist of some bug fixes along with improvements to xrefs. These include:

• Adding a check before the Mapping step to ensure that we have object_xref rows
• Fixing a bug in DumpXref that was keeping ambiguous peptides in the xref dumps (causing exonerate to fail)
• Added changes to handle getting descriptions from the HGNC file and adding them onto Uniprot_gn xrefs
• Removed the CCDS source from the list of sources for vertebrates and plants
• Added new ZFIN file to download as well as changed priority of ZFIN parsing
• Removed get_xref_mapper method which is used for second pipeline and causing problems for now

Benefits

Bug fixes and changes to improve quality of xrefs.

Possible Drawbacks

If applicable, describe any possible undesirable consequence of the changes.

Testing

• Have you added/modified unit tests to test the changes?
• If so, do the tests pass?
• Have you run the entire test suite and no regression was detected?
• TravisCI passed on your branch

Dependencies

If applicable, define what code dependencies were added and/or updated.