Fix color scheme for 2D heatmap.
Allow to split matching to W with additional information.

Introduces a new parameter 'additional info' for histograms, which contains
either a list (x value, optional y value), or a dictionary with 'x', 'y' as
(optional) keys.

This can be used to split the W into W from H or top, and other W in gen
matching plots.
Enter v51.
Fix histogram filling.
Refactor Process a bit.
Move SetSumw2 to the right spot (this fills histograms with proper errors).
Implement event unification.
Split W bin properly.