Radiation for production

[tulioricci]

No merge, only production feeder

Questions for the review:

• [ ] Is the scope and purpose of the PR clear?
  • [ ] The PR should have a description.
  • [ ] The PR should have a guide if needed (e.g., an ordering).
• [ ] Is every top-level method and class documented? Are things that should be documented actually so?
• [ ] Is the interface understandable? (I.e. can someone figure out what stuff does?) Is it well-defined?
• [ ] Does the implementation do what the docstring claims?
• [ ] Is everything that is implemented covered by tests?
• [ ] Do you see any immediate risks or performance disadvantages with the design? Example: what do interface normals attach to?

[majosm]

Took a quick pass, looks great to me. I just have a little concern about how we're handling fluid_bc_class, not sure it'd be critical at this point.

Thanks! (FYI, the "real" radiation PR is #893. This one is that + ESDG. Strictly speaking I guess that means ESDG should merge before radiation? But I think if we merge #893 into main, then merge this into #922, and finally merge that up with the new main, it should work out in the end.)