venelin/PCMBase: Some new functionality and bug fixes
Added function PCMTreeInsertTipsOrSingletons.
Added possibility to specify metaI as a character string (function name) rather.
So far, it was only possible to specify this as a function object or as the list
resulting from calling the function on the tree, data and PCM model.
Added an operator + for PCMTree objects. This is analogical to the operator +
for phylo objects but maintains the edge.part and part.regime members
Removed argument parametrizations from PCMGenerateModelTypes. Now it's possible
to specify the type of parametrization (all/default) separately for each model
Added automatic generation of White noise model types.