added a quick_check to enable quick heuristic check to allow rapid processing of genomes where there are no alternative transcripts

Ordering only by the session's created date failed to realise we could have more than one entry for the same stable id. Ordering now by the score as a way of getting rid of this ambiguity. Issue was picked up by Rhoda running the code on an older MySQL version which consistently exhibited different behaviour to later MySQL versions

get_alt_attrib_codes was added as a fresh dependency. This module missed it. We return nothing for this a we don't have alt_attrib codes for variations