[FEATURE] plot_ratio() to support Weighted histograms

[andreypz]

Describe the problem, if any, that your feature request is related to
I believe the current plot_ratio() method does not take into account weights of the histograms when calculating the errors.

Describe the feature you'd like

Propagate errors correctly into the ratio uncertainty, taking into account the weights

Describe alternatives, if any, you've considered

Using coffea.hist.plotratio instead

[heatherrussell]

Hi, I just encountered this - can a warning at least be added to the documentation that says that weights will be ignored in the calculation of the ratio? It's not obvious at all that they are ignored and this could easily lead to / have already led to incorrect conclusions/analyses.