Merge #963

963: Add tests and documentation with improvement of downcast type compatibility (part of #845) r=hgrecco a=jthielen

As a part of #845, this PR adds tests for downcast type compatibility with Sparse's `COO` and NumPy's `MaskedArray`, along with more careful handling of downcast types throughout the library. Also included is new documentation on array type compatibility, including the type casting hierarchy digraph by @shoyer and @crusaderky.

While this PR doesn't fully bring Pint's downcast type compatibility to a completed state, I think this gets it "good enough" for the upcoming release, and the remaining issues are fairly well defined:

- MaskedArray non-commutativity (#633 / numpy/numpy#15200)
- Dask compatibility (#883)
- Addition of CuPy tests (no issue on issue tracker yet)

Because of that, I think this can close #845, but if @hgrecco you want that kept open until the above items are resolved, let me know.

- [x] Closes #37; Closes #845
- [x] Executed ``black -t py36 . && isort -rc . && flake8`` with no errors
- [x] The change is fully covered by automated unit tests
- [x] Documented in docs/ as appropriate
- [x] Added an entry to the CHANGES file


Co-authored-by: Jon Thielen <github@jont.cc>

CHANGES

@@ -4,6 +4,13 @@ Pint Changelog
 0.10 (unreleased)
 -----------------
 
+- Documentation on Pint's array type compatibility has been added to the NumPy support
+  page, including a graph of the duck array type casting hierarchy as understood by Pint
+  for N-dimensional arrays.
+  (Issue #963, Thanks Jon Thielen, Stephan Hoyer, and Guido Imperiale)
+- Improved compatibility for downcast duck array types like Sparse and Masked Arrays. A
+  collection of basic tests has been added.
+  (Issue #963, Thanks Jon Thielen)
 - Improvements to wraps and check:
   - fail upon decoration (not execution) by checking wrapped function signature against
     wraps/check arguments.
@@ -12,7 +19,7 @@ Pint Changelog
     (might BREAK code not conforming to documentation)
   - when strict=True, strings that can be parsed to quantities are accepted as arguments.
 - Add revolutions per second (rps)
-- Improved compatbility for upcast types like xarray's DataArray or Dataset, to which
+- Improved compatibility for upcast types like xarray's DataArray or Dataset, to which
   Pint Quantities now fully defer for arithmetic and NumPy operations. A collection of
   basic tests for proper deferral has been added (for full integration tests, see
   xarray's test suite). The list of upcast types is available at