refactor: refactor internal plottable histogram and allow comparision of plottable histograms (for pulls, ratio, ...)

[cyrraz]

The main goal goal of this PR is allow the comparison of histograms for creating ratio plots, pull plots, ...

• The first step to achieve this goal is the introduction of the EnhancedPlottableHistogram class, inheriting from the UHI protocol.
• The second step is the definition of the function ensure_plottable_histogram() that can convert ROOT, numpy, boost_histogram, Hist into an EnhancedPlottableHistogram.
• The third step (still a work in progress) is to copy and adapt from plothist the comparison functions so that they take EnhancedPlottableHistogram objects and propagate the uncertainties. The comparison functions will use ensure_plottable_histogram() on the input histograms so that they can compare varied histogram types.
• A minor change is the addition of a noxfile.py allowing to test `ROOT inputs.

This PR will be un-drafted when third step in completed.