Fix to VSI track association
This MR will fix an issue in the VrtSecInclusive algorithm. The fix will change the track attachment step, so that each track is attached to the vertex before the algorithm attempts to attach another track. Without this fix, some secondary vertices only have 1 attached track, when they should have more. The issue is described in detail here (https://gitlab.cern.ch/atlas-phys-susy-wg/RPVLL/DisplacedVertices/athena/-/issues/4), which includes plots showing the impact of this fix on secondary vertices in some MC and data events.