Fix mistake in call to partial function

[cphyc]

PR Summary

This bug was reported by @claytonstrawn on our channel. It is due to a minor typo in the way art deals with particle velocity fields which caused a Python error.

Steps to reproduce:

import yt
ds = yt.load_sample("D9p_500")
ad = ds.all_data()
ad["particle_velocity_x"]

PR Checklist

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

[cphyc]

At the moment, the test(s) are not triggered at all. I'm working on it but in the meantime I'm switching to draft.

[cphyc]

@matthewturk @brittonsmith @neutrinoceros this is ready to go (I just need to bump the answer tests for Jenkins). However, it seems #3087 would require bumping the answer tests again. My question is thus: should we fix #3087 in this PR before merging or submit the answer tests knowing the velocities may be off?

[matthewturk]

I think whatever gets this in and closed is best.

[neutrinoceros]

We lost the report, I need to rerun tests
@yt-fido test this please

[munkm]

I think it's fine to not fix #3087 in this PR. We can do a follow up PR for that, and now that we have the submodule for answers I don't think it's a huge issue to have lots of answer-store commits. Unless I'm missing something?

[neutrinoceros]

I don't think it's a huge issue to have lots of answer-store commits. Unless I'm missing something?

FWIW, I think it's already hard enough to properly handle a single PR there at a time, I don't think anyone wants to abuse them if it can be avoided. But if we need 2 updates instead of one, that's fine by me :)