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. 
The fill func was for the case the EcalRegressionData becomes a member variable, you might want to just allocate it once rather than re-allocating all the time. Thinking about it, if you're having issues with this approach, then I'll just put it as a member variable and stop re-allocating it all the time. Not a big object so I wouldnt have thought it matters too much either way.