New configuration setting slice_source_kwargs

[forman]

Addresses #82. To close, see #86.

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 (8dc8167) to head (9835d42).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #84   +/-   ##
=======================================
  Coverage   99.93%   99.93%           
=======================================
  Files          30       30           
  Lines        1436     1443    +7     
=======================================
+ Hits         1435     1442    +7     
  Misses          1        1           

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[thomasstorm]

Please address comment on guide.md