Fix EELSArctan gradients

[ericpre]

Same as hyperspy/hyperspy#3388.

Progress of the PR

• Fix EELSArctan gradients,
• Update link to EDS demos in user guide,
• fix API reference of some components,
• [n/a] update user guide (if appropriate),
• [n/a] added tests,
• add a changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• Check formatting of the changelog entry (and eventual user guide changes) in the docs/readthedocs.org:exspy build of this PR (link in github checks)
• ready for review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.49%. Comparing base (770f06f) to head (41acbcf).
Report is 84 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #55      +/-   ##
==========================================
+ Coverage   87.77%   88.49%   +0.72%     
==========================================
  Files          67       67              
  Lines        7287     7286       -1     
  Branches     1179     1179              
==========================================
+ Hits         6396     6448      +52     
+ Misses        626      568      -58     
- Partials      265      270       +5     

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[jlaehne]

Just some minor suggestions for better readability of some of the docstrings.

[francisco-dlp]

All good, except that the changelog does not include the documentation improvements.