Histograms from multiple sources

[alexander-held]

As brought up by @kratsg:

There are applications where a template histogram needs to be filled with data from various sources, with possibly different selection requirements / weight per source. An example for this is a histogram of a Monte Carlo sample, containing an expected distribution of data across multiple years. Across those years, the luminosity might differ (so different weights are needed per year), as might the selections to be applied (due to e.g. a change in trigger threshold). Native support for this would be good.

[alexander-held]

While there is no native support for this yet, #78 implements a method to achieve this manually in the meantime. A user-defined function can be provided to cabinetry to handle this.