2D cell-level reweighting of egamma objects

Implemented code by Mykola Khandoga (after cleanup/indentation/variable renaming/..) for 2D cell-level reweighting.

Original code here: https://gitlab.cern.ch/mykhando/athena/tree/rew

Also made a lot of cleanup, added comments, simplified code, removed useless parts, moved from iostream to Athena messages, renamed class members to match ATLAS guidelines