Follow-up: DUTAssociation -> historam hNoAssocCls should have entries = number of tracks

Merged Katharina Dort requested to merge kdort/corryvreckan:fix_hNoAssocCls_histo into master

This MR solves the issue about the discrepancy between number of tracks and entries in the hNoAssocCls histogram which Jens discovered. I opened a separate MR since I cannot push into the MR which Jens opened.

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