feat: support rebinning in histogram postprocessing

[alexander-held]

Adding functionality to re-bin during template post-processing.

to-do:

• decide whether to support index- or coordinate-based rebinning (or both?)
• understand typing of histogram re-assignment
• converge on config format
• tests
• figure out relation to Binning config setting: this is only cosmetic (for plots) for histogram inputs; for ntuple inputs it would no longer correspond to the actual binning needed in plots if rebinning is used -> automatically convert internally?

resolves #412