Format/syntax:
* Refactored variables, functions and methods to snake_case. Use of CamalCase is know reserved to classes.
* Changed docstring style to be that used in numpy/scipy.
Modifications:
* BEAST2XML.to_string: returns the xmls tree as a string.
* BEAST2XML.to_xml: writes the xmls to a file.
New features:
* BEAST2XML.change_parameter_state_node: Change the values of the stateNode for a parameter.
* BEAST2XML.change_prior: Change the values of a parameters prior. Only tested for the Uniform distribution.
* BEAST2XML.add_initial_tree adds initial Newick tree to BEAST2 xmls.
* BEAST2XML.add_rate_change_dates: Add specific dates for parameter changes in skyline models.
* BEAST2XML.add_rate_change_times: Add specific times for parameter changes in skyline models.