Allow building electrons from tracks with 0 pixel

Following the discussion today (23/09/2020)

This is for the standard electrons. But is there to support future LRT workflows.

These do not pass any of our std selections, so we had dropped them (AOD space reasons). But seem useful for LRT.

Mentiong @amorley , @mdanning , @goblirsc ,@jpoveda ,@jmaurer , @turra , @gunal , @npetters , @ludovica

I hope I do not miss anyone from the discussion.

The summary is we do slide 7 https://indico.cern.ch/event/957859/contributions/4026454/attachments/2107042/3543706/Egamma_discussion.pdf

BUT the pink box, goes under the blue rather than the red. As we do not want to mix collections (requires C++ changes). So we keep things Std -> Std and LRT -> LRT . Do not need/want to do Std->Std with 1 pixel and LRT+Std (0 pixel only) -> LRT.