initial checkin of tree similarity code.
tree similarity -- continue development
Add unit test for similarity
ongoing similarity development
tree: perf improvement, bug fix

It was printing a blank space iat end; t shouldn't.
It was attempting to iterate over somethingthat has only on item;
that's just a waste of cpu cycles.
combo_tree: fix parsing bug

zero-ary functions were being mis-parsed.
More combo-ant vocabulary linking fixes
trim trailing whitespace from combo strings

Now that the combo tree printer doesn't sloppily add a whitespace ...
At last .. some semblance of tree-parsing sanity
Expand the tree-flatening work.
Expanded similarity measurement
Implement an actual tree similariyt measure
don't use a new directory for this code.
Merge pull request #772 from linas/simi

Combo tree similarity measures