Added a single warning for running without a CCDS database, and provided an exit for certain no-data situations.

Tests null cases and basic functioning of sorting algorithm.

New utility object for finding canonical transcripts. Refactoring of "set_canonical_transcript.pl" logic.

added method remove_source_priorities; it's called from xref_mapper.pl when option -recalc_display_xrefs is used

changed option -recalc_display_xrefs to boolean, it wasn't possible to recalculate display xrefs without using the xref db

slight memory requirement adjustements as well

set_display_xrefs deleted, no need to override this method as gene and transcript display xrefs are set separately in DisplayXrefs.pm

can be run across several hosts at a time

[ENSCORESW-220]. Being removed in favor of our new generate_default_aliases.pl code for the production DB.

delete records from display_xref_priority and gene_desc_priority when status is reset to parsing_finished or mapping_finished

added option run_all to run even if there has been no change to the species

added tables display_xref_priority and gene_desc_priority - they are used in DisplayXrefs.pm http://www.ebi.ac.uk/panda/jira/browse/ENSCORESW-219

changes to set_display_xrefs and set_gene_descriptions http://www.ebi.ac.uk/panda/jira/browse/ENSCORESW-219