Topic fix mem leak

After a few investigations, I found out that the memory leak was coming from the DiPhotonProducer and it was due to the PhotonIdUtils::pho4MomCorrection, where the photon candidate was cloned and then returned by value.

After the fix, I run on 50k events with an asymptotically stable resident memory size of 800MB and a total size of 1.2GB.
I still observed an overall increase of the memory size by a factor of 2 between the beginning and the end of the run, but that may be due to the buffer allocation. In fact the increase seemed to be step-wise.