Plot tasks that summarize categories

[riga]

We should have generic tasks within columnflow that produce plots that don't show a variable distribution in a single category, but that show several categories at once.

I see two possible use cases:

1. A plot that has a one bin per category on the x-axis, with process yields per category being stacked. This is a standard summary plot to convey category compositions and statistics.
2. A plot that shows the distribution of a variable for a single process but for different categories. This could be used to make quick category comparisons.