A few improvements related to the Model class

[paulromano]

This PR makes a few small improvements related to the openmc.Model class:

• When calling mode.run, you can now pass keyword arguments on to export_to_xml or export_to_model_xml, which importantly allows you to call model.run(remove_surfs=True), which was not possible before.
• When no <plots> are present in a model.xml file, OpenMC will now look for a plots.xml file. This closes plot_inline function doesn't work well when using model.xml #2446.
• Updated an error message regarding missing settings.xml which might be a little confusing (if you are using model.xml, it doesn't matter that you're missing settings.xml)

[jtramm]

This seems like a good way of fixing the issue. The keyword arguments when exporting to XML seem useful as well.

LGTM!