Searches for a better (more parsimonious) tree by performing
alternating cycles of 'Nearest Neighbour Interchange' and
'K.L.' optimizations.
NOTES
The current parsimony value (number of base changes needed) is
displayed after the 'Current Par' prompt in the upper part of
the 'ARB_PARSIMONY' window and the 'Message' window (pops up
while calculating).