New configuration setting extra

[forman]

Closes #82

Also reworked section Slice Sources in documentation.

Checklist (strike out non-applicable):

• Changes documented in CHANGES.md
• Related issue exists and is referred to in the PR description and CHANGES.md
• Added docstrings and API docs for any new/modified user-facing classes and functions
• Changes/features documented in docs/*
• Unit-tests adapted/added for changes/features
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.93%. Comparing base (db3ab06) to head (2a23449).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #86   +/-   ##
=======================================
  Coverage   99.93%   99.93%           
=======================================
  Files          30       30           
  Lines        1443     1446    +3     
=======================================
+ Hits         1442     1445    +3     
  Misses          1        1           

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[forman]

Thanks 👍