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
  (experimental).
• Removed argument parametrizations from PCMGenerateModelTypes. Now it's possible
  to specify the type of parametrization (all/default) separately for each model
  type.
• Added automatic generation of White noise model types.
• Improved man pages.