add gen card for radiative decays of Z boson

+ add gen card for radiative decays of Z boson
+ requires #4433  (which in turn is a fw-port of an already integrated PR #1101), where the filter to select only radiative decays is developed
+ it would be very handy and helpful having this gen card in the release, in order to generate relval samples w/o inclusion of local cards; it's the case for the d.v.mc exercise