phylo.distance now using itertools

[CHANGED] use the standard library combinations function from itertools instead
    of my own implementation.
code tidy in phylo/distance.py

[CHANGED] really private attributes and methods are no longer really private.
    There was no need for it.
phylo/distance refactor

[CHANGED] producing name combinations is now a separate function that just uses
    the itertools.combinations function.
[CHANGED] producing estimates of pairwise distances from triad lengths is now a
    separate function. This allows calling by user client code.
[NEW] added getAllParamValues method on EstimateDistances class. This returns a
    copy of the results dictionary.