Rename UDF to UDFStep for clarity, and remove from root namespace

[rlamy]

Follow-up to #23

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[shcheklein]

thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.45%. Comparing base (e05463f) to head (e751d18).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #88      +/-   ##
==========================================
- Coverage   84.46%   84.45%   -0.01%     
==========================================
  Files          94       94              
  Lines        9511     9510       -1     
  Branches     1883     1883              
==========================================
- Hits         8033     8032       -1     
  Misses       1150     1150              
  Partials      328      328              

Flag	Coverage Δ
datachain	84.39% <100.00%> (-0.01%)	⬇️

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