Making dEtaIn seed available for ID purposes

This pull request simplifies the calculation of the new ID variable dEtaInSeed presented recently in the e/gamma meeting. As the name suggests, it is dEtaIn but computed using the eta of the seed cluster rather than the supercluster. It is simply dEtaIn-sc eta + seed eta. I have added a function to GsfElectron which makes it, it would be nice if we could have this, it just simplifies things and avoids mistakes but we can live without.

I have also added a class to the ID framework which can cut on this variable.