Add test for plotting with DAGMCUniverse.plot

[pshriwise]

Description

Add a test for plotting a DAGMC universe with the Python API Universe.plot method.

[paulromano]

...which would fail currently. The problem is that the way we currently do this under the hood is by creating a model with a geometry based on that universe, but what is missing on the model are the materials corresponding to the DAGMC cells. Unfortunately, the same problem occurs on Geometry.plot and Model.plot. In principle Model.plot could work because the Model has the material information, but right now it just defaults to calling Geometry.plot which calls Universe.plot.

A partial solution is to move the main plotting logic to Model.plot, which could then be called by Geometry.plot and Universe.plot instead of the other way around. This doesn't solve the problem of missing materials for Geometry.plot and Universe.plot but at least Model.plot would work for a DAGMC model then.

[pshriwise]

Ah, too true. I was getting a little ahead of myself here.

In principle, once #3056 is complete we could make DAGMCUniverse.plot viable when the DAGMCCell information has been populated by openmc.lib... and we should add a test for it once enabled 😄. This does currently require a Model object to exist however, so maybe it's a moot point in the end. At the very least it would allow the logic for plotting to remain where it is for the time being.