[WIP] Branch length estimation

[sprillo]

I migrated my branch length estimation code here, under cassiopeia/tools. This PR should be see as a big draft to get discussions started around APIs. For example, note that my APIs currently depend on a Tree class, rather than on networkx.DiGraph.

Note also that my code is totally self-contained right now, segregated from any utilities that Cassiopeia might already provide, such as ancestral state reconstruction via maximum parsimony (I have a method for this in the Tree class which I just implemented de-novo).