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Faster cell copying for HeavyIons

Walter Lampl requested to merge wlampl/athena:HICellCopy into 23.0

During Heavy Ion reconstruction, the CaloCell container is deep-copied and modified to to subtract energy from each cell (underlying event). The copy-process turns out to be surprisingly slow. A callgrind-profile identifies the frequent allocation of small amounts of memory by new'ing each LAr/Tile cell for each event as a hotspot. DataPools are a well-known remedy for this problem.

This MR introduces a new cell-copy tool that benefits from DataPools for LAr and Tile cells and omits the some unused functionality (and thus complexity) of the previously used tool. The HICaloCellCopier is now about 20% faster.

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