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

ENSCORESW-2725: can run for species, taxon or division

Bugfix/official naming

ENSCORESW-2827: minimal test on xref config validity

ENSCORESW-2819: clean up steps in mapping

ENSCORESW-2818: remove vega from xref code

[ENSCORESW-2820] Update links in pull request template

registry_public_cores.t now uses frozen test database

...the difference being that now we use the database server configured
in MultiTestDB.conf rather than the public one. Note that since the
Registry scans all databases (modulo version number) on the server and
does not know about test prefixes, there might be some warnings
about duplicate species/group combinations; these should ultimately be
harmless though and as long as the test-database schema version matches
that of the API, the tests should succeed.

Had to rewrite the "registry knows more than 10 DBAs" test to "registry
knows at least 1 DBA" because by default that is what MultiTestDB
provides.

See-also: https://www.ebi.ac.uk/panda/jira/browse/ENSCORESW-2325

backport ENSCORESW-2767: multi-species support for protein feature

ENSCORESW-2810: pass RFAMParser even if no xrefs found

Do not call fetchrow_hashref() if the statement has been exhausted

update to latest schema

ENSCORESW-2740: fetch correct xref