RegionGrower - add option to clear singletons from the tree

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The RegionGrower analyses typically end up with many of the elements forming a 
large polytomy at the root level.  These singletons are terminal nodes which 
are direct descendents of the root node and are the leftovers from the analysis 
which were not needed to maximise the objective index.  (This does not normally 
happen when minimising).

While they are essentially harmless, they do make interaction with the tree 
more difficult.  

It would be useful if users were given the option to remove these from the 
tree.  In a perfect world it would be at any time, but for now it can be an 
option in the region growing process.

Original issue reported on code.google.com by shawnlaffan on 13 Apr 2014 at 6:42