Tests for variables with units (pydata#3654)

* add tests for variable by inheriting from VariableSubclassobjects

* make sure the utility functions work with variables

* add additional tests for aggregation and numpy methods

* don't assume variables have a name attribute

* properly merge the used arguments

* properly attach to non-xarray objects

* add a test function for searchsorted and item

* add tests for missing value detection methods

* add tests for comparisons

* don't try to check missing value behaviour for int arrays

* xfail the compatible unit tests

* use an additional check since identical is not sufficient right now

* check for compatibility by comparing the dimensionality of units

* add tests for fillna

* add tests for isel

* add tests for 1d math

* add tests for broadcast_equals

* add tests for masking

* add tests for squeeze

* add tests for coarsen, quantile, roll, round and shift

* add tests for searchsorted

* add tests for rolling_window

* add tests for transpose

* add tests for stack and unstack

* add tests for set_dims

* add tests for concat, copy and no_conflicts

* add tests for reduce

* add tests for pad_with_fill_value

* all and any have been implemented by pint

* remove a unnecessary call to np.array

* mark the unrelated failures as xfail

* remove a debug print

* document the need for force_ndarray

* make is_compatible a little bit more robust

* clean up is_compatible

* add docstrings explaining the use of the method and function classes

* put the unit comparisons into a function

* actually squeeze the dimensions separately