Fix issue with zn.nodes

[PythonFZ]

Improve tests and bugfix

[codecov]

Codecov Report

Merging #542 (d104c19) into main (16f895e) will decrease coverage by 0.27%.
The diff coverage is 80.64%.

@@            Coverage Diff             @@
##             main     #542      +/-   ##
==========================================
- Coverage   90.16%   89.90%   -0.27%     
==========================================
  Files          19       19              
  Lines        1220     1228       +8     
  Branches      442      441       -1     
==========================================
+ Hits         1100     1104       +4     
- Misses        119      123       +4     
  Partials        1        1              

Flag	Coverage Δ
fulltest	88.43% <80.64%> (+0.07%)	⬆️

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Impacted Files	Coverage Δ
zntrack/fields/zn/__init__.py	94.17% <73.91%> (-1.80%)	⬇️
zntrack/fields/field.py	92.53% <100.00%> (+1.01%)	⬆️

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