Adding nascent source site plot capability.

[pshriwise]

This feature is a little rough still but it adds the ability to plot source sites on top of the current view. All sites are plotted whether they're on the slice plane or not. There are a number of things I'd want to add (see below) before incorporating this, but it has a significant amount in place so I thought I'd create a draft PR at least.

See utilization in openmc-dev/openmc#2857

TODO:

• Toggle for source site visibility in the main dock
• Toggle filter of visible souce sites based on coincidence with slice plane
• Arrows for source locations
• Colormap bar for particle energy

[shimwell]

Nice to see this PR, I remember a few people where asking for this on the slack channel a while back.

I'm keen to mirror this functionality on the python api if that is ok

[shimwell]

Perhaps a variable could be added for number of source samples (n_samples)

[pshriwise]

I built this out a little bit today and I'm pretty happy with it now. The animation below shows an annular CylindricalIndependent source with R=1 and Z=[0,10]. Main additions include the ability to toggle visibility, change the color, and filter source particles out that are too far from the slice plane.

[pshriwise]

A couple of features to be added yet I suppose, but I'd call this a good base for the feature and wouldn't mind seeing it merged as-is

[paulromano]

@pshriwise I ended up just forging ahead and implementing the changes we talked about today. Let me know if you're happy with how it looks/functions and if so, we can go ahead and merge. Thanks for adding this feature!