rewrite quantify and dequantify (#17)

* add a function to extract / retrieve unit attributes

* add a function to zip mappings

* partially rewrite Dataset.pint.quantify

* add tests for the unit attribute extraction function

* also make sure that deleting attributes works

* add a function to attach units as string attributes

* also extract unit attributes from variable objects

* refactor the unit attr extraction tests

* allow using a different attr name

* use the conversion functions to implement dequantify

* update the docs on the return value

* update the conditions for _decide_units

* rewrite the Dataset quantify and dequantify methods

* raise on unit stripped warnings

* remove the test checking that dequantify raises if there were no quantities

* make sure Dataset.pint.quantify raises only if we would overwrite a quantity

* don't try to put units in indexes

* check the attributes after dequantify more thoroughly

* make sure the attributes are strings

* move the str conversion of pint.Unit to a util function

* update the dequantify docstrings

* update the quantify docstrings

* add tests for Dataset.pint.dequantify

* update the parameter spec of Dataset.pint.quantify

* remove the old attach and quantify functions

* fix the template used to format accessor attributes

* remove the attrs on quantify

* make the description of the units kwargs easier to understand

* mention the format of the units

* don't use rst references in the descriptions in docstrings

* warn about loading data into memory while quantifying

* don't try to quantify indexes

* replace the delete parameter with a strip function

which is important because modifying in-place makes it difficult to test.

* update the docstrings

* more docstring updates

* use K instead of degK

Co-authored-by: Jon Thielen <[email protected]>

Co-authored-by: Jon Thielen <[email protected]>

Author: keewis

docs/api.rst

@@ -21,7 +21,7 @@ DataArray
 ---------
 .. autosummary::
    :toctree: generated/
-   :template: autosummary/accessor_property.rst
+   :template: autosummary/accessor_attribute.rst
 
    DataArray.pint.magnitude
    DataArray.pint.units