WIP: 22.0-refactor-TrackCollectionMerger

This MR splits the current dual-function TrkTrackCollectionMerger into its components: a 'pure' collection merger which combines collections, and a Track Summary updater. The pure merger writes an intermediate combined collection into storegate which is then retrieved by the updater and the track summary is updated. It will no longer be necessary to 'merge' collections when there is only one collection, and the track summary updater can now be tweaked separately. InDetRecExample has been modified to accommodate the now two-step process (with no further optimisations). A simple unit test has been included for the merging step, with an additional testing algorithm (which simply stuffs collections in storegate) to prepare the test. Making a WIP becasue my git skills are lacking and I may committed more than I intended. work undertaken in the context of ATLIDTRKCP-184.