implement scale_factor/add_offset CF conformance test, add and align tests

[kmuehlbauer]

• Closes #xxxx
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

This is adding conformance tests for scale_factor/add_offset/dtype when encoding/decoding cf packed data with

xarray/xarray/coding/variables.py

Line 217 in c75ac8b

class CFMaskCoder(VariableCoder):

. This doesn't include tests for _FillValue/missing_value/valid_range etc. which need a separate test.

The idea behind is two-fold:

• be strict when encoding to only write CF conforming data, raise Errors if non-conforming
• be liberal when decoding, but issue warnings

This is another step to move forward to finally fix issues like #2304 and a bunch more.

[kmuehlbauer]

OK it seems this is ready for a first round of reviews.

A bit of added context:

Currently there is no dedicated function for checking for CF standard conformance. The idea is to read as much as possible also non-standard conforming data files, but restrict writing non-standard conforming files.

The implemented function ensure_scale_offset_conformance takes a strict keyword argument, which is True when encoding and False when decoding. If strict=True it will raise errors if there is a mismatch with the standard and when strict=False it will issue warnings.

I've only had to adapt a few tests which where not conforming to standard on encoding to align with that. I've observed some warnings in the test suite which we might to have a look into.

One idea would be to fix erroneous scale_factor/add_offset with our best fitting estimate. This is already done for list-type scale_factor/add_offset.

I will follow-up with checks for CFMaskCoder.

[kmuehlbauer]

Setting status back to draft for now, still evaluating solutions for the CF encoding/decoding.

[kmuehlbauer]

I'll close this one. Might resurrect some of the ideas here down the road.