Adapt TrigGepPerf to use GepCellHandlerTool as cell preparation step
This merge request rearranges the code that is already in the TrigGepPerf package and changes what was previously the CaloCellHandlerTool into the standalone GepCellHandlerTool and adds the GepCellMap class. The advantage is that cell collections only need to be prepared once and are then stored in StoreGate for retrieval by downstream algorithms. Previously, every algorithm had to prepare their own cell map.