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

• [x] Add some functionalities back to public API,
• [x] Rename private modules to start with underscore to reduce their visibility and make more clear that they are not part of the public API,
• [x] docstring updated: there were still a lot of leftover from hyperspy,
• [x] update user guide: there were still a lot of leftover from hyperspy,
• [n/a] added tests,
• [x] add a changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• [x] 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)
• [x] ready for review.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 77.01318% with 157 lines in your changes missing coverage. Please review.

Project coverage is 88.62%. Comparing base (3b7425e) to head (9e0d0be). Report is 48 commits behind head on main.

Files with missing lines	Patch %	Lines
exspy/_misc/eels/tools.py	40.93%	95 Missing and 6 partials :warning:
exspy/_misc/material.py	83.78%	11 Missing and 13 partials :warning:
exspy/_misc/eds/utils.py	92.59%	7 Missing and 9 partials :warning:
...py/_misc/eels/electron_inelastic_mean_free_path.py	59.25%	11 Missing :warning:
exspy/material.py	80.00%	1 Missing :warning:
exspy/misc/eds/utils.py	85.71%	1 Missing :warning:
exspy/misc/eels/tools.py	85.71%	1 Missing :warning:
exspy/utils/__init__.py	80.00%	1 Missing :warning:
exspy/utils/eels.py	83.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #59 +/- ## ========================================== + Coverage 88.55% 88.62% +0.06% ========================================== Files 67 76 +9 Lines 7341 7411 +70 Branches 1187 1192 +5 ========================================== + Hits 6501 6568 +67 - Misses 571 574 +3 Partials 269 269 ```

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[CSSFrancis]

@ericpre I can review this today!

[ericpre]

Thank you @CSSFrancis for the review. I added a shim for the functionalities that have been added to the public API. Do you want to have another look?