BUG: Make sure fields are read in the order they appear in the file

[cphyc]

PR Summary

This fixes #4880.

The issue was that the reader would assume fields were passed in the order they appeared on file, so that depending on the order in which one would load the data, we'd get different results.

[matthewturk]

This looks like it might fix bugs ... were there any?

[cphyc]

@matthewturk yes, this fixes the bug reported in #4880 (which IMO is a major one).

Essentially, when yt detects field dependences, it creates a list of on-disk fields in an arbitrary order. This then gets passed to the reader, which (wrongly) was assuming the fields were passed in the order the appear in the file.

In #4880, yt tries to read density, metallicity, pressure but the fields appear on-disk in the order density, pressure, metallicity. As a consequence, pressure and metallicity end up being swapped. This PR fixes this by ordering the fields in the order they appear on disk and voilà.

[neutrinoceros]

Hum. This looks simple enough but I'm not comfortable to merge a fix for a major bug without at least an attempt at automated testing. It probably not possible to prove the software is always correct, but a simple regression test seems feasible ?

[cphyc]

I'll give it a try, but so far I only was able to trigger it with a somewhat massive simulation (few 10GiB).

[matthewturk]

I think the likelihood of a problem is low enough that this should go in. We do similar logic elsewhere, and this seems absolutely correct.

[neutrinoceros]

Fair enough. @cphyc, suit yourself !

[neutrinoceros]

(to be clear, I'm not worried that the patch is incorrect, I'm worried that a similar bug might creep back in at some later point in the future and we have to pay the full price of figuring it out again)

[cphyc]

I've added a test (972ccec) and checked locally it correctly fails against main (2880d15).

FYI, I've added a nose test rather than a pytest one because it appears that pytest-based tests don't use actual data…

[neutrinoceros]

I'd suggest to move this inside the test function for clarity

[neutrinoceros]

I checked the test fails on main and pass with this branch, thanks a lot ! Feel free to ignore my minor remark and merge if you will, otherwise I'm happy to approve again later today.