Clean up of core/peakfit/fit_wrapper.cpp

I'm not sure whether you have seen my reply on the corresponding issue. While looking at this, I thought I saw more than just the ambiguous method names which needs to be cleaned up or could be simplified. Some of this additional changes are in necessary - like removing an unimplemented method from the class declaration, others are more a question of style and taste. You can find the details for these changes here.

Resolves issue #49 (closed)