Randomisations - add tree perturbation option

[GoogleCodeExporter]

The randomisations currently work only on the basedata (although see recent 
work on group properties - issue #388).  

A useful addition would be to randomly perturb the positions of nodes on any 
trees used as analysis arguments.  

In the first instance this would work only on the terminals, randomly 
reassigning terminal nodes across the tree while maintaining the topology and 
branch structure.  

Further variants could randomly re-assign any named nodes (for cases where 
internals are named).  Later work can also randomly reassign branch lengths.  

Note:  Need to add references for this sort of work. as it has been implemented 
in other systems.

Original issue reported on code.google.com by shawnlaffan on 13 Jul 2013 at 3:56