Simplify using custom slice sources

[forman]

Using custom slice sources is now simplified by not forcing slice sources to return a SliceSource instance.
Instead, slice source functions should be allowed to return a SliceItem.

• Rewrite tests, add new, implement changes
• Rename argument slice_obj into slice_item to underline it is one of the elements of the slices iterator passed to zappend, renamed type SliceObj into SliceItem
• Update (simplify) user guide, especially confusing section about slice source
• Dropped concept of slice factories entirely, removed type SliceFactory and functions to_slice_factory, to_slice_factories.

Closes #78

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 99.92%. Comparing base (249b7e8) to head (643fc07).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #80   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files          26       30    +4     
  Lines        1396     1419   +23     
=======================================
+ Hits         1395     1418   +23     
  Misses          1        1           

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