refactor towards datatree

[forman]

[Description of PR]

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 99.46%. Comparing base (2983a52) to head (04f1873).
Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #62   +/-   ##
=======================================
  Coverage   99.46%   99.46%           
=======================================
  Files          67       67           
  Lines        2806     2810    +4     
  Branches      412      413    +1     
=======================================
+ Hits         2791     2795    +4     
  Partials       15       15           

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