new capabilities for validating identifiability of rate matrices

[NEW] added include_root argument to Tree.getEdgeVector. Controls whether the
    root edge is returned.
[NEW] added two methods on cogent.evolve likelihood function objects. These are
    to be used to evaluate whether the Markov model is statistically
    identifiable.
[NEW] a new function for conservatively checking the uniqueness of a rate
    matrix

Patch provided by Ben Kaehler, thanks Ben!