Dev further photon preselection

This pull request contains :
- the end of the preselection story (I think)
- a couple of fixes (and cleanup) to classes.h and classes_def.xml to get closer to instructions from https://twiki.cern.ch/twiki/bin/view/CMSPublic/SWGuideCreatingNewProducts

I have created a new constructor for the diphoton candidates, since I could not manage to get around storing the modified photons in the events... Maybe there is a better way to go ?