Stop converting jupyter to python after class definition is found

[PythonFZ]

Currently the full notebook is converted into a *.py file.
With this change you can add # zntrack: break to stop converting the Notebook into a *.py file, if the Node that the conversion triggered was already found.

[codecov]

Codecov Report

Merging #238 (b42c1af) into main (2622e8d) will decrease coverage by 0.42%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #238      +/-   ##
==========================================
- Coverage   69.86%   69.44%   -0.43%     
==========================================
  Files          23       23              
  Lines         979      985       +6     
  Branches      180      183       +3     
==========================================
  Hits          684      684              
- Misses        278      284       +6     
  Partials       17       17              

Flag	Coverage Δ
unittests	69.44% <0.00%> (-0.43%)	⬇️

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Impacted Files	Coverage Δ
zntrack/core/jupyter.py	17.94% <0.00%> (-3.27%)	⬇️

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