Phylomatic: add 'random placement' function

[camwebb]

Goal

Rather than collapsing a node in which some taxa are not directly matched, fit the unplaced taxa into the tree at random points

Algorithm

1. In existing, non-polytomous clade, add unmatched taxon at a random node

Rationale

Need to explore this: is attaching a taxon to a random inner node the appropriate action? Test it:

1. Make random tree
2. Pick one terminal taxon at random
3. For the node N to which this taxon attaches, output the depth D of N and node number N, and total number of nodes T in the tree
4. Repeat from (1) 1,000 times
5. Hopefully the distributions of D and N/T are both uniform