[asdf/xarray.DataSet|DataArray] fix serialization of attributes

[marscher]

Changes

The wrapping of Variables inside the DataSet serialization did not contain the attrs property. Additionally, the coordinates
of a DataArray can also enclose attributes, these were also not being serialized.

Refactored some for loops in favor of list comprehensions.

Related Issues

Closes #349

Checks

• updated CHANGELOG.md
• updated tests
• updated doc/
• update example/tutorial notebooks

[codecov]

Codecov Report

Merging #384 (5c8a07e) into master (1999c8d) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #384      +/-   ##
==========================================
- Coverage   97.44%   97.43%   -0.01%     
==========================================
  Files          86       86              
  Lines        5196     5183      -13     
==========================================
- Hits         5063     5050      -13     
  Misses        133      133              

Impacted Files	Coverage Δ
weldx/asdf/tags/weldx/core/common_types.py	100.00% <100.00%> (ø)
weldx/asdf/tags/weldx/core/data_array.py	100.00% <100.00%> (ø)
weldx/asdf/tags/weldx/core/dataset.py	100.00% <100.00%> (ø)

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[marscher]

Funnily the asdf tests pass on Python-3.9, so I guess the dataclassses.field(defaultfactory=dict) thing behaves differently on 3.8...

[marscher]

Ah this is the schema tests failing. Sry. So do I need to adopt the schema as well, indicating the new optional "attrs" field should be there?
Please enlighten me @vhirtham, @CagtayFabry

[vhirtham]

Thanks for the cleanups. Really funny to see all the bad lines of code that I wrote when I started with Python 😄