Simplify interface for curve fitting algorithms

[Anders-Markvardsen]

Currently any curve fitting algorithm is required to implement in particular:

virtual void function(const double* in, double* out, const double* xValues, const double* yValues, const double* yErrors, const int& nData) = 0;

where yValues and yErrors refer to the data you are fitting against.

function() currently returns differences between observed and calculated data.
Change function() to just return calculated data and remove yValues and yErrors
from the argument list of this function.

Do the same for functionDeriv().

[Anders-Markvardsen]

This issue was originally trac ticket 905