Analyze "branchiness" of version trees.

[bbengfort]

Run a postprocessing script that computes the number of branches in the version tree for each object.

[bbengfort]

See #85

[bbengfort]

I've created a Tree data structure that can be loaded from versions to perform this analysis. I've also created an iPython notebook that explores several types of trees:

• Chain: every node has only one child except the last, which has zero.
• Prong: a small chain is forked at the end by two equal length branches.
• Trident: a small chain is forked at the end by two equal length branches and one smaller branch.
• Thorns: a chain where every other child has an additional child.
• Cactus: a chain where every other child has a branch with two nodes.
• BTree: a tree where every node has two children except the leaf nodes.
• Fan: the worst case scenario, no node has a child except the root.

To discuss their various properties with regards to the "branchiness" metric.

[bbengfort]

Current metrics for comparison are:

• mean of leaf depths / height of tree from root.
• number of branches
• overall height of the tree relative to maximal height of chain.