Ray object dl for SPH data: interpolate kernel table for (b/hsml)**2 instead of b/hsml

[nastasha-w]

Fixes a (possible) bug in the calculation of path lengths dl for SPH/non-grid data in the YT Ray objects. This affects e.g., column density and absorption spectrum calculations, like those done in the Trident package.

PR Summary

Replace
dl = itab.interpolate_array(b / hsml) * mass / dens / hsml**2
by
dl = itab.interpolate_array((b / hsml)**2) * mass / dens / hsml**2
in the _generate_container_field_sph method of the YT Ray object (https://github.com/yt-project/yt/blob/main/yt/data_objects/selection_objects/ray.py).

This is the proposed fixed for the possible bug mentioned in issue #4781.
In short, the _generate_container_field_sph retrieves path lengths dl for normalized SPH kernels by linearly interpolating values calculated for a smallish set of (impact parameter / smoothing length) values. However, if I'm not mistaken, the table actually stores those dl values for different values of (impact parameter / smoothing length)^2. That means we need to input (b / hsml)**2 into the interpolation function, not (b / hsml).

PR Checklist

• New features are documented, with docstrings and narrative docs
• Adds a test for any bugs fixed. Adds tests for new features.

The only documentation issue I can see here is that if this was indeed a bug, we should probably send out some message warning people who might have used/be using Ray objects for papers about the issue. As for tests, @chummels metioned a possible test on the slack channel involving SPH-ifying a grid dataset and comparing the surface/column densities retrieved from both.

[welcome]

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[matthewturk]

I believe that the analysis in #4781 is correct! Thank you for your detailed report, @nastasha-w -- this is a great find, and very appreciated. I am going to mark approve, but I think for a change of this magnitude (even if it is just one line!) we do need another set of eyes.

[neutrinoceros]

This is completely out of my expertise so I'll leave it to others to review; I just wanted to ask to anybody who'd want to push the button to please triage this to 4.3.1 or 4.4.0 depending on the expected impact ! thanks !

[nastasha-w]

@matthewturk Thanks! It agree it's best to be sure about this. I did add a test for Ray objects to this pull request. I don't think it's entirely complete; for one thing, the mass / density factor in the dl calculation is always 1 in the test mock dataset I used. However, it does pass on this branch and fail on the main branch (2 main commits ago anyway).

The new test is in test_rays.py: test_ray_particle2(). It just uses a bunch of asserts, and returns None if all tests are ok. Is there anything else I need to do to incorporate this into automatic testing?