ENSCORESW-2553: retrieve correct accession for peptides

ENSCORESW-2792: one UCSC parser per species

ENSCORESW-2792: separate mouse UCSC to avoid clashes

Test module for xref parsers, test WormbaseDirectParser

ENSCORESW-2744: remove dependent_xref when removing master

Remove deprecated Variation methods

+ XrefParser::Database stores a DBConnection, setters delegate to it
+ New test database, for xrefs
+ Setup to test XrefParsers
+ Tests for WormbaseDirectParser

Update Bio::EnsEMBL::Exon->is_coding():

Feature/xref config cleanup

- Update documentation on required parameters
- Throw an error if a Transcript isn't given
- Add a test for regression