Improve cluster sorting in segment-finding

The MuonClusterSegmentFindingTool sorts clusters by layer in order to quickly access clusters in particular layers. This sorting is mainly by global z: however, as multiple clusters can be from the same layer, the final output is a vector of vector of clusters. This simplifies the previous sorting by first using stable_sort to sort by global z position. Then the clusters are looped through and assigned to vectors for each layer. This should improve the performance compared to the previous sorting, which, if there were n clusters, required looping through the list of clusters n times. Diff-root showed no changes on 50 events from the ART Run-3 symmetric layout test.

Closes ATLASRECTS-6362