Restructure Custom Abundance Widget code

[yuyizheng1112]

This PR aims to restructure the code of custom abundance widget as mentioned in #1777.

Description

• Added CustomAbundanceWidgetData class to store the data which will be edited in the widget.
• Used data attribute, a CustomAbundanceWidgetData instance, to organize data in CustomAbundanceWidget and DensityEditor.
• Generated the CustomAbundanceWidget instance from a CustomAbundanceWidgetData instance.
• Edited docstrings to fix docstring warnings in documentation building.
• Fixed step graph for newly added element.

Motivation and context

Since the widget has integrated more information than before, the separation of the data and the widget enables the module more organized and easier to be developed and maintained. Also, it would be easier to implement tests as well as new TARDIS model in the future.

How has this been tested?

• Testing pipeline.
• Other.

Examples

Type of change

• Bug fix.
• New feature.
• Breaking change.
• None of the above.

Checklist

• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
  • (optional) I have built the documentation on my fork following the instructions.
• I have assigned and requested two reviewers for this pull request.

[tardis-bot]

Before a pull request is accepted, it must meet the following criteria:

• Is the necessary information provided?
• Is this a duplicate PR?
  • If a new PR is clearly a duplicate, ask how this PR is different from the original PR?
  • If this PR is about to be merged, close the original PR with a link to this new PR that solved the issue.
• Does it pass existing tests and are new tests provided if required?
  • The test coverage should not decrease, and for new features should be close to 100%.
• Is the code tidy?
  • No unnecessary print lines or code comments.

[codecov]

Codecov Report

Merging #1784 (39d735f) into master (575002c) will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1784      +/-   ##
==========================================
- Coverage   58.42%   58.35%   -0.07%     
==========================================
  Files          66       66              
  Lines        6684     6702      +18     
==========================================
+ Hits         3905     3911       +6     
- Misses       2779     2791      +12     

Impacted Files	Coverage Δ
...s/tardis/visualization/widgets/custom_abundance.py	14.43% <0.00%> (+0.59%)	⬆️

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[jaladh-singhal]

LGTM - thanks for restructuring it.

The notebook works fine too, the error messages just don't show up correctly - not sure if it's related to this?

[jaladh-singhal]

I think it breaks add shells functionality