[RF] Enable passing of gradient function directly to RooMinimizer

After my previous work on the RooFit minimizer interfaces, it is
relatively easy to allow passing a custom gradient function to the
RooMinimizer constructor.

The interface might still evolve in the future, but this commit is still
a milestone because now we can get actual `RooFitResult` objects that
were obtained minimizing a RooFuncWrapper that wraps both the likelihood
and the gradient code.

Some helper functionality that was originally in the new RooFuncWrapper
class but that is not needed anymore with the new minimizer interface
was removed.