expose testing functions as public API

[keewis]

As suggested in #11, this exposes assert_units_equal as public API (as pint_xarray.testing.assert_units_equal). I don't think we have more of those, but if we do they should be added here, too.

[codecov]

Codecov Report

Merging #24 into master will increase coverage by 0.33%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
+ Coverage   86.91%   87.25%   +0.33%     
==========================================
  Files           7        9       +2     
  Lines         688      706      +18     
==========================================
+ Hits          598      616      +18     
  Misses         90       90              

Impacted Files	Coverage Δ
pint_xarray/__init__.py	100.00% <100.00%> (ø)
pint_xarray/testing.py	100.00% <100.00%> (ø)
pint_xarray/tests/test_testing.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 226f747...52e9e87. Read the comment docs.

[keewis]

the docstring could use a review, but otherwise this is ready for review and merge

[jthielen]

Generally looks good to me, just a couple small questions/suggestions on the docstring.