Clean up unit test for dscim.utils.utils.c_equivalence

[brews]

Cleans up unit test for dscim.utils.utils.c_equivalence().

• Refactors the single test for dscim.utils.utils.c_equivalence() into multiple tests, with each test checking for a single behavior/test case.
• The expected outcome for each test was precalculated and stored explicitly with some precision rather than calculated dynamically when tests are run. The expected values were created using the same math and code in the original test. For example, expected = (np.divide(array ** (1 - 10), 1 - 10).values.mean() * (1 - 10)) ** (1 / (1 - 10)) is now just expected = xr.DataArray(np.array(2.32634448)), which is the answer I get on my machine.
• The test now checks against expected outcomes with xr.testing.assert_allclose(actual, expected) rather than assert expected == actual, as the original had done. This is helpful because == checks between floats can be difficult, if not impossible, and unreliable. Using xr.testing.assert_allclose allows for some "fuzziness".

[brews]

We're using xr.testing.assert_allclose with the default rtol=1e-05 and atol=1e-08. I'm not sure if we need to change how tight the tests are.

[codecov]

Codecov Report

Merging #135 (a6ad90a) into main (95ec0de) will not change coverage.
Report is 9 commits behind head on main.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #135   +/-   ##
=======================================
  Coverage   65.30%   65.30%           
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  Files          17       17           
  Lines        1842     1842           
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  Hits         1203     1203           
  Misses        639      639           

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[kemccusker]

looks good from my perspective, except for the minor update of the "normal eta" language