Add IDPVM hit plots versus mu
This MR adds a few additional validation plots to InDetPhysValMonitoring
, by allowing to monitor the track hit / hole / dead content as a function of pileup.
Also clean up the constructor of the hit plot class a bit by ensuring appropriate default initialisation of member histograms at declaration time.
CC @vcairo @npetters @sroe @goetz as IDPVM regulars.