Reduce code duplications

[forman]

Reduce code duplications for operation classes that can be customized: rules, processors, formatters.
Fixed #21.

Closes #21

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 97.15%. Comparing base (7803dc0) to head (57e0976).
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
+ Coverage   96.17%   97.15%   +0.97%     
==========================================
  Files          51       52       +1     
  Lines        2065     2108      +43     
  Branches      270      270              
==========================================
+ Hits         1986     2048      +62     
+ Misses         46       32      -14     
+ Partials       33       28       -5     

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