Implement surface flux tallies

[GuySten]

Description

This PR enable openmc to calculate surface flux tallies.

Fixes #2.

Theory

https://www.tandfonline.com/doi/pdf/10.13182/NSE09-72

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[GuySten]

The tests does not pass because of #3743.

[GuySten]

This PR is ready for review.

[pshriwise]

Thanks for the PR @GuySten! I'm wondering what your thoughts are on providing the $\mu$ cutoff and ratio values on SurfaceFitlers themselves. As indicated in the paper linked in the description, different factors may be appropriate for different surfaces (for example, if the surface is interior or a boundary surface). Thus, it might make sense to allow these to be applied to SurfaceFitler's themselves. When scoring, I think it would be pretty clean to look up the SurfaceFitler object using Tally:get_filter and adjust the score as needed depending on the value of $|\mu|$ computed above the tally loop.

Given that the score requires a tally have a SurfaceFilter and that it isn't meaningful to have more than one SurfaceFilter on a tally (though interestingly we do currently allow it).

Let me know what you think!

[GuySten]

If I remember correctly there is a case when there are only CellFrom and Cell filters without surface filters.

I think if there is interest we can add in a subsequent PR attributes to the Surface objects themselves that will override the defaults in the setting object.

[pshriwise]

If I remember correctly there is a case when there are only CellFrom and Cell filters without surface filters.

I think if there is interest we can add in a subsequent PR attributes to the Surface objects themselves that will override the defaults in the setting object.

Ah, yes, right you are. Part of the motivation for my comment was that we avoid additional global settings, but in light of the CellFrom/Cell case you mentioned it seems the best approach.

[pshriwise]

Haven't forgotten about this. Will give it another look tomorrow

[GuySten]

@pshriwise, are you reviewing this PR?

[paulromano]

Thanks a lot for implementing this @GuySten! A few comments below that shouldn't be too difficult to address:

[GuySten]

I thought about another problem.
current has direction so it makes sense to tally partial directed currents from one cell to the other.
But flux has no direction so the usage of either CellFilter or CellFrom filter will tally only particles in one direction when we actually need to also tally particles in the other direction.

I will think about a way to fix that so particles in the other direction will also score.

EDIT:
I fixed this problem by using a virtual particle with current cell and last cell switched and using the of the two particles scores in surface flux score.

[GuySten]

@paulromano, I addressed all your concerns.