remove None values form asdf tree by default

[CagtayFabry]

This is to reproduce the old ASDF behavior pre asdf-format/asdf#863
Some tutorials seem to run now 👍

Also, some utilities provided by boltons could be useful for further tree manipulation
https://sedimental.org/remap.html#drop-empty-values
https://boltons.readthedocs.io/en/latest/iterutils.html#nested

Notes

• We should discuss if we want this as default behavior
• we can remove the drop_none_attr wrapper from weldx.asdf.utils with this (done)

Changes

• default wrapper for to_tree now also removes None values

Related Issues

Closes # (add issue numbers)

Checks

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

[CagtayFabry]

this should work with any ASDF version so I think we should include this in master and rebase here once ready @vhirtham

this branch -> master -> asdf28

[codecov]

Codecov Report

Merging #212 (796f0af) into master (af7a78b) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #212      +/-   ##
==========================================
- Coverage   99.73%   99.72%   -0.01%     
==========================================
  Files          69       69              
  Lines        3719     3694      -25     
==========================================
- Hits         3709     3684      -25     
  Misses         10       10              

Impacted Files	Coverage Δ
weldx/asdf/utils.py	100.00% <ø> (ø)
weldx/asdf/__init__.py	100.00% <100.00%> (ø)
weldx/asdf/tags/weldx/aws/design/base_metal.py	100.00% <100.00%> (ø)
weldx/asdf/tags/weldx/aws/design/connection.py	100.00% <100.00%> (ø)
...dx/asdf/tags/weldx/aws/design/joint_penetration.py	100.00% <100.00%> (ø)
weldx/asdf/tags/weldx/aws/design/sub_assembly.py	100.00% <100.00%> (ø)
weldx/asdf/tags/weldx/aws/design/weld_details.py	100.00% <100.00%> (ø)
weldx/asdf/tags/weldx/aws/design/weldment.py	100.00% <100.00%> (ø)
weldx/asdf/tags/weldx/aws/design/workpiece.py	100.00% <100.00%> (ø)
...asdf/tags/weldx/aws/process/arc_welding_process.py	100.00% <100.00%> (ø)
... and 17 more

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