Ecal regression refactor

This code refactors the ECAL regression inputs so a new single class gets the data rather than the same (buggy) code being copy & pasted over 3 files. It also fixes a divide by zero bug. It changes the behaviour of the pat::Photons but for the better in my opinion. 
I think I'm going to leave it unless you really think there is a  problem. Changing to a  constructor based approach will take some time and its really not clear to be right now its worth it. May revisit for 73X.