Associated arbitrary grid edge unit registry with dataset unit registry for issue #2087

[jcoughlin11]

PR Summary

This PR associates the unit registry of the YTArray edge in _sanitize_edge with that of the dataset that the arbitrary grid is being constructed for. This is needed because if the user passes non-dataset units for the edge, i.e.

ag = ds.arbitrary_grid(left_edge = [0,0,0] * yt.units.kpc,
    right_edge = [1,1,1] * yt.units.kpc,
    dims = [32,32,32])

then when yt attempts to get the data, there was a UnitParseError. This fixes that error.

Issue #2087

PR Checklist

• Code passes flake8 checker
• New features are documented, with docstrings and narrative docs
• Adds a test for any bugs fixed. Adds tests for new features.

[ngoldbaum]

Nice!

My only comment is that there's a lot of repeated code in the tests. I think you could rewrite it as a for loop like this:

for le, re, le_answer, re_answer in zip(les, res, le_answers, re_answers):
    ag = ds.arbitrary_grid(left_edge=le, right_edge=re, dims=dims)
    assert np.array_equal(ag.left_edge, le_answer)
    assert np.array_equal(ag.right_edge, re_answer)
    assert ag.left_edge.units.registry == ds.unit_registry
    assert ag.right_edge.units.registry == ds.unit_registry
    ag['density']

Where les, res, le_answers, and re_answers are lists or arrays of the all of the inputs and answers you're expecting.

Writing it like this is more maintainable since it makes it easier for future editors to change one section of the test without also forgetting to update the other sections.