Mention source of elements database in docstring

[ericpre]

As mentioned in #85 (comment).

Progress of the PR

• Mention source of elements database in docstring,
• docstring updated (if appropriate),
• 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 88.70%. Comparing base (ee31d0a) to head (0f9d7c0).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #87   +/-   ##
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  Coverage   88.70%   88.70%           
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  Files          77       77           
  Lines        7453     7453           
  Branches     1200     1200           
=======================================
  Hits         6611     6611           
  Misses        572      572           
  Partials      270      270           

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[jlaehne]

Chantler reference is already mentioned in other docstrings, but way of mentioning is quite inconsistent.

[ericpre]

Chantler reference is already mentioned in other docstrings, but way of mentioning is quite inconsistent.

@jlaehne, is it better now with the references?

[jlaehne]

Chantler reference is already mentioned in other docstrings, but way of mentioning is quite inconsistent.

@jlaehne, is it better now with the references?

Maybe align also the citation in the Bibliography of the User Guide (use DOI, it currently uses the subtitle and not the title of the page).