Update observer tool to keep each rejection location throughout ambiguity solving

As the logic within the ambiguity solver was changed between r21 and r22, the observer tool was only picking up the last rejection reason, potentially missing the salient information about track rejection. This was ok in r21, where each rejection/modification of a track would create a subtrack, but in r22 subtracks are not necessarily created when adding/removing hits etc. This MR updates the track observer tool to keep each rejection location a given subtrack may get.

Tagging @gfacini @vcairo @dmclaugh