Unbiased hits

Changed the HPD occupancy histograms to take into account the events without hits, removing an annoying bias. Tested on data in collisions, with the laser and with dark counts, appears to do what was expected. For completeness plots: https://indico.cern.ch/event/477523/contributions/2167902/attachments/1272770/1886922/hits_per_hpd_bias_fix_comparison.pdf