add cell container to EGAM7
As requested here: https://its.cern.ch/jira/projects/ATLEGAMDPD/issues/ATLEGAMDPD-145 add calorimeter cell information in EGAM7 mainly for machine learning developments (such as DNN/CNN/GNN). The size increase is O(10%) This derivation is produced only once a while and having cells information would be useful for the aforementioned studies.