fix feature that at start of each new run MET histograms are filled twic...

When running privately our modules over data we realized the BookHistogram method is called for each run. We didn't anticipate that in our code (we thought bookHistograms would be called once at the begin of each job). This lead to a feature which blew up the number of times we fill MET histograms per even when running over a large number of runs. We introduce now a protection which should fix that feature. It seems though when running centrally this feature was not present.