Improve the SWIG API to the Fit module

The SWIG API to the Fit module is revised completely to make the minimizer's call to Python functions and conversion of results more explicit. Auxiliary functions are added to call a Python callable with a Parameters instance and convert the result to double or a C++ std::vector as required.

The Python extension to the Minimizer class is updated and improved.

The unneeded PyCallback C++ class and ParametersIterator Python class are removed.