plot_inline function doesn't work well when using model.xml #2446

The openmc.plot_inline function accepts one or more Plot objects and produces an image to be displayed in a Jupyter notebook. However, the way it works is by locally creating a plots.xml file under the hood. If the model you're trying to plot is stored as a single model.xml file, it will bypass the plots.xml file altogether, and hence OpenMC is not aware of the plots it should be producing.