Feature/multiple model support for residual hist on comparer

[ryan-kipawa]

Trying to residual plot on a comparer with more than one model currently fails. This PR adds a test for that and a fix. The test is also relevant for other plot functions that return multiple ax objects.

Hoping to merge this as a precursor to #456 🙃

[ecomodeller]

Works as expected.

...
cc = ms.match(obs=[o1, o2, o3], mod=[mr1, mr2])
cc[0].plot.residual_hist();

cc[0].sel(model="SW_2").plot.residual_hist();