Adding typemap to convert String to const openstudio::path&

This should be completely backwards compatible, I don't think we should provide an out typemap so openstudio::path values are returned as String.  I also don't know if we want to advertise this as the preferred way to write code, I would continue recommending using OpenStudio::Path if possible.

@lefticus can you take a look at the the typemap?  Should we also provide a type map for "openstudio::path" and "const openstudio::path"?

@nllong this is a present for you