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
They represent two different states: Initialised but never filled, Initialised, but no seed present from which to take variables.  In the first case the electron should fail pixel match selections (hence not zero), in the second case it's less clear so Sam and I chose to make the electron pass the selections (hence zero.)  This can be harmonised if necessary.