Adapt names of multiplicity histograms in EventLoader type modules

Plots of the number of pixels per Corryvreckan event are named in most event loader type modules as pixelsPerFrame, but the title of the histogram is "Pixel multiplicity per Corry frame". This could be unified.

My idea is to title these histograms as "Pixel multiplicity per Corry event", and name these histograms in the code and ROOT file as pixelMultiplicity.