Skillgrid ecdf

[ecomodeller]

Ecdf is a way to compare the skill of two (or more) models across many observation locations, very common in hydrology (Kratzert et al, 2019.

This PR is an attempt to add this functionality to gridded skill.

Example:

plot() delegates to xarray, so how do I add plot.ecdf()🤔

[jsmariegaard]

I guess we would need to implement a plotter class for SkillGrid then. We could wrap xarray plotter but also add our own plotting functions like ecdf