tests for datasets with units (#3447)

* start writing the tests for dataset

* add tests for initializing Datasets

* add tests for aggregation methods / functions

* add tests for the ndarray methods / properties

* add tests for missing value handling methods

* add tests for comparison methods

* add tests for reordering / stacking

the test for to_stacked_array seems a bit brittle

* add tests for indexing methods

* remove the commented out xfail on Dataset.squeeze

* add tests for head, tail and thin

* add tests for the computation methods

* add tests for grouped operations

* add tests for the content manipulation methods

* fix reindex_like to actually expect errors where appropriate

* use DataArray.copy to replicate a DataArray with different data

* add tests for repr / str

* remove the comment about moving the merge tests

* construct a new data array instead of using `copy`

which in combination with `assign_coords` make preserving `MultiIndex`
instances much more complicated.

* update whats-new.rst

doc/whats-new.rst

@@ -104,7 +104,7 @@ Internal Changes
 ~~~~~~~~~~~~~~~~
 
 - Added integration tests against `pint <https://pint.readthedocs.io/>`_.
-  (:pull:`3238`) by `Justus Magin <https://github.com/keewis>`_.
+  (:pull:`3238`, :pull:`3447`) by `Justus Magin <https://github.com/keewis>`_.
 
   .. note::