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. 
yeah I saw that when I was porting the code, fine I changed it to be the deltaR2 internally and updated methods as a result.