[RF] Add Barlow-Beeston tutorial and clean up related classes.

- Add a tutorial for the Barlow-Beeston method, rf709_BarlowBeeston.C.
- Remove legacy iterators from RooHistConstraint and RooParamHistFunc,
which are used to implement the Barlow-Beeston method.
- Replace iterative logGamma in RooHistConstraint by std::lgamma
- Make sure that bins with zero entries don't get touched by MINUIT.
This reduces the number of degenerate parameters without any effect, and
stabilises the covariance matrix.
- Update documentation of the respective classes.