Internally import from unyt

[neutrinoceros]

PR Summary

Quoting CONTRIBUTING.rst

• Internally, only import from source files directly -- instead of:
  from yt.visualization.api import ProjectionPlot
  do:
  from yt.visualization.plot_window import ProjectionPlot

I think this principle could and should be applied to unyt, which we currently wrap under yt.units.
Directly importing from unyt instead of the wrapper would reduce the overhead of this wrapper: it's easier to learn the origin of a function/class if we avoid layered import. This should reduce the maintenance burden of the wrapper as well, though the primary goal is to make definitions easier to discover from everywhere else in yt/

highlight points

• This should not remove anything from namespaces yt.units (and its children) in any way, so as not to break any downstream code.
• Documentation (website + docstrings) remains unchanged, this is not about telling users not to use the wrapper, but only a matter of clarifying the code base.

[neutrinoceros]

This is the one proposal originally part of YTEP0037 that didn't reach consensus. It is not considered part of that YTEP any more.

[neutrinoceros]

@yt-fido test this please

[neutrinoceros]

this is in direct conflict with #2777, I'll remove this after #2777 is merged

[matthewturk]

I'm having a hard time finding it, but we retained the display_ytarray stuff, right?

[neutrinoceros]

you mean this ?

yt/yt/units/__init__.py

Line 74 in 9138a0a

def display_ytarray(arr):

Yup, still there. This PR removes nothing, its one and only purpose is to replace indirect imports by direct ones.
There is indeed a handful of functionalities that are still defined in yt.units, but with this branch it is much easier to spot them.

[munkm]

For reviewers of this PR -- Here's a discussion we had about internally importing from unyt in the code styling ytep (links to the first comment in a thread):

yt-project/ytep#14 (comment)

[munkm]

I have some thoughts about doing this; I will weigh in when I get back early Sept.

[neutrinoceros]

Note that this project

https://github.com/sqlalchemyorg/zimports

Can be used to perform this change in a semi-automated way by

1. replacing all imports from yt.units with star imports from unyt
2. running zimports to expand explicit required imports

[neutrinoceros]

If I'm ever reattempting this again, I'd rather start fresh. It's dishearthing to see how many conflicts this has been accumulating in less than a year.