Fix when multiple Hough clusters are processed for the same X-Z track

The variable nXHits should be a const since it is the same for every loop corresponding to a different Hough cluster candidate. If a hit of one of the X layers is removed in the hit cleaning procedure, the count on X hits should be reset for the next Hough cluster candidate.

This bug fix has negligible effect on physics performance, however it does change the references. The efficiency and ghost rates are expected to go up slightly.

Reference updates in Brunel!984 (merged) and Moore!438 (merged)