First stage commit only. Partial functionality (detailed below), but works and reduces
chance of missing problems.

1. Made exception data structure richer for check_for_stops to allow a number of different
types of exception to to be expressed.

2. Added a new exception type, that of known TGA-stopping Sec-coding transcript.

3. Added data-structure to add_selcys stop checking to allow specification of known
TGA-stopping Sec-coding transcripts.