Merge branch 'new_egamma_histograms' into '21.0'

New histograms for the egammaValidation package - ART

See merge request !25799

(cherry picked from commit 5a1f5655)

2cde005d first try
924ecd5e adding new variables
da363aa0 cleaning a bit
a6fe204e adding changes
d4c96177 skip histograms with zero entries