[Cin] Histogram plugin

[Andrea paz]

I tried to make some clarifications on the Histogram plugin (and some
minor changes in other plugins as well). Most importantly, I added a
mathematical introduction to the Theory section, also adding a new
image, to provide more detailed context. But I did not touch GG's old
explanations of "sum vs max". Not having fully understood them, I
don't want to edit them so as not to make a mistake.

Besides the images and Plugins.tex I also attach the txt version if
anyone wants to correct or improve it.
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