Saving pixel match info in the electron for offline ID purposes

This pull request adds the pixel seed into to the GsfElectron object to enable better electron - photon disambiguation and miss matched tracks at the identification level. This change has been agreed by E/gamma (Lindsey, Matteo,Sam) and it would be nice if we could include it into 73X
ROOT is often very permissive and doesn't complain about missing dictionaries either until absolutely needed.