Fix restoring Gosh and Hartree Slater GOS

[ericpre]

As noticed in hyperspy/exspy-demos#3 (comment), the eels core loss edge are not restored correctly. This is because subshell_factor was missing from the whitelist and therefore its value wasn't restored correctly.

Progress of the PR

• Add subshell_factor to the whitelist of EELS core loss edge component,
• [n/a] docstring updated (if appropriate),
• [n/a] update user guide (if appropriate),
• 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 88.45%. Comparing base (eb65577) to head (df90ec6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
+ Coverage   88.07%   88.45%   +0.38%     
==========================================
  Files          67       67              
  Lines        7276     7295      +19     
  Branches     1179     1179              
==========================================
+ Hits         6408     6453      +45     
+ Misses        591      573      -18     
+ Partials      277      269       -8     

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