Allow a user to ask for overlaying signal normalised to total background

[MoAly98]

On a stacked histogram, user may want to overlay signal(s) histogram(s) on top as lines that show the signal scaled up to the total background yield (to make it visible).

This should be an option to add for all plots, an appropriate sample should be created with a rescaled histogram and when a Stackatino is created it should have a Fill = False style.