typo fix
Add Newick_gtree.expand_multifurcation_to_bifurcation.
Adding Convex.find_b_assignments.
Fixing EnumFuns.combinations's behavior when n < r.

Previously, this would cause an error. Now, it returns an empty enum.
Adding a partial implementation of multifurcating convexify.

Strict convexity is, at the moment, not supported. That's the next (and
hopefully last) thing to implement before this is done.
Major cleanups and comment revisions in convex.

Some cruft in find_b_assignments and _build_apartl wasn't actually necessary
with the new implementation. This removes that cruft.
Finish strict convexity; add more tests.
Document strict convexity more.
Merge pull request #273 from matsen/273-multif-convex

fix convexify for multifurcating trees