Optionally write sequence position in right-hand column of clustal files

Followon from JAL-2997 - Clustal files may optionally have the sequence number for the right-most residue in each sequence written in the right-hand column of the alignment. JAL-2997 dealt with a problem with the parser, but the Clustal file writing code was not updated to support generating files in this revised format.

This isn't needed for Core jalview functionality, but if there's a need please let us know so we can reprioritise this enhancement !