Vtk exporter fix to save more than one product

[olastrz]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.90%. Comparing base (48ea037) to head (70c0523).
Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
PySDM/exporters/vtk_exporter.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1439      +/-   ##
==========================================
+ Coverage   84.60%   84.90%   +0.29%     
==========================================
  Files         375      375              
  Lines        9248     9252       +4     
==========================================
+ Hits         7824     7855      +31     
+ Misses       1424     1397      -27     

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