Plot source sites using opnemc.lib.sample_exeternal_source

[pshriwise]

As part of the latest NEA OpenMC workshop, we plotted source sites on top of the geometry and tally information. We did this using the openmc.Tracks class, but I don't see why we couldn't do the same here with the openmc.lib.sample_external_source feature.

It would provide an somewhat interactive to provide users some assurance that their source is defined correctly.