Draft: Implement the computation of the "mirror cell" for pile-up corrections
This MR implements two new functors "MirrorID" and "MirrorEnergy" to be applied on neutrals. These functors can be used to compute the pile-up energy corrections in the ECAL. See Chap 5 of https://cds.cern.ch/record/2886547?ln=en for more details.
Goes together with LHCb!4504.
Edited by Camille Normand