group and pcoa modules use dm class
Complete overhaul of DistanceMatrix class.

Instead of inheriting from ndarray, DistanceMatrix now holds an ndarray as a member and delegates calls to the ndarray (http://stackoverflow.com/a/4759140).  Implementation is much simpler and easy to follow, and we still have the ndarray interface for free.
Added __mul__ method.
Added SampleIds getter/setter, more tests.