Vtk/vtk improvements

[xavArtley]

I need to use VTK panel at work so in the same time I had some small enhancements

Now nans are handle for VTKVolumes and nan_opacity in slices can be set

I updated model with the new bokeh syntax in static init

[codecov]

Codecov Report

Merging #2826 (48aed06) into master (0773b0d) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2826      +/-   ##
==========================================
- Coverage   83.06%   83.05%   -0.02%     
==========================================
  Files         188      188              
  Lines       24305    24419     +114     
==========================================
+ Hits        20188    20280      +92     
- Misses       4117     4139      +22     

Impacted Files	Coverage Δ
panel/models/vtk.py	98.50% <100.00%> (+0.02%)	⬆️
panel/pane/vtk/vtk.py	97.07% <100.00%> (+<0.01%)	⬆️
panel/widgets/tables.py	85.14% <0.00%> (-1.42%)	⬇️
panel/widgets/select.py	93.82% <0.00%> (+0.01%)	⬆️
panel/tests/widgets/test_tables.py	99.43% <0.00%> (+0.03%)	⬆️
panel/models/tabulator.py	94.52% <0.00%> (+0.07%)	⬆️

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[philippjfr]

Ready to merge?

[philippjfr]

Oh if you get a chance document the parameter in the reference gallery.