Improved facilities for initialising from known trees

This release improves the MultiTypeTreeFromNewick class used for initialising chains from known typed trees. It also includes an additional class MultiTypeTreeFromUntypedNewick which initialises the untyped portion of the typed tree using a Newick string and simulates type changes down this using MTT's uniformization algorithm.

Instructions for using these classes and doing fixed tree analyses can be found on the wiki.