Structure colouring from disulphide features not localised to cysteines

The featurerenderer code was hacked in order to make features called 'disulphide bond' render on an alignment as a pair of regions at the start position and the end position of the feature. If a structure is associated with that sequence, then the disulphide feature colours the whole stretch of peptide from start to end rather than just the first and last residues.


****** ADDITIONAL INFORMATION ******

this may not be trivial to fix.