Add plots modify

[PythonFZ]

• tests
• support zn.plots
• allow multiple plots from the same data source (consider https://dvc.org/doc/user-guide/project-structure/dvcyaml-files#plots) over dvc plots modify. Maybe via y = [loss, mae]
• requires DVC 2.53.1 or later
• move template to ./dvc_plots/templates so it can be tracked by the project - if file path is outside the repo?

[codecov]

Codecov Report

Merging #581 (6a154e6) into main (ff14c26) will increase coverage by 0.69%.
The diff coverage is 97.56%.

@@            Coverage Diff             @@
##             main     #581      +/-   ##
==========================================
+ Coverage   88.42%   89.11%   +0.69%     
==========================================
  Files          20       20              
  Lines        1365     1443      +78     
  Branches      510      564      +54     
==========================================
+ Hits         1207     1286      +79     
+ Misses        158      157       -1     

Flag	Coverage Δ
fulltest	89.11% <97.56%> (+0.69%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
zntrack/fields/dvc/__init__.py	88.52% <80.00%> (+3.77%)	⬆️
zntrack/fields/field.py	95.77% <98.64%> (+3.23%)	⬆️
zntrack/fields/zn/__init__.py	91.69% <100.00%> (+0.03%)	⬆️

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