Replace RooFIter with suggested RooAbsCollection iterators
The copy constructor was deleted in a recent ROOT version. This commit will fix the nightly errors such as lhcb-head/... by using the recommended RooAbsCollection itertaros.
I was unable to test these changes though, because I have no experience with the DecRateCoeff.cxx class (but use the DecRateCoeff_Bd.cxx through the runSFit_new.py scrip...). So if any expert has scripts available that actually use this implementation, a quick check to see if everything works as expected would probably be good.