Extend public API and fix documentation

[ericpre]

Add back some functions which were removed when splitting from hyperspy. Some modules and functions that were not to the public API as listed in the API reference have been removed.

Progress of the PR

• Add some functionalities back to public API,
• Rename private modules to start with underscore to reduce their visibility and make more clear that they are not part of the public API,
• docstring updated: there were still a lot of leftover from hyperspy,
• update user guide: there were still a lot of leftover from hyperspy,
• [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

Attention: Patch coverage is 96.29630% with 3 lines in your changes missing coverage. Please review.

Project coverage is 87.80%. Comparing base (0f39dab) to head (df1702b).

Files	Patch %	Lines
exspy/material.py	80.00%	1 Missing ⚠️
exspy/utils/__init__.py	80.00%	1 Missing ⚠️
exspy/utils/eels.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #59      +/-   ##
==========================================
+ Coverage   87.77%   87.80%   +0.03%     
==========================================
  Files          67       70       +3     
  Lines        7286     7301      +15     
  Branches     1179     1179              
==========================================
+ Hits         6395     6411      +16     
+ Misses        626      625       -1     
  Partials      265      265              

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