Lazy reader imports

[akaszynski]

We did an OSS scan within pyansys/pyfluent and discovered a few vulnerabilities. One of those was a TIFF reader from VTK, and it's entirely possible to simply remove the offending library from vtk.

The only issue with this is that we import the library on import, which inspired me to consider if we can just lazy import most, if not all, of our libraries since we now require VTK v9 or greater.

For now, this PR just lazy imports vtkmodules.vtkIOImage.vtkTIFFReader, but is there any reason why we can't just do it for all the modules? This has several advantages:

• We only import what we use. If you are only post-procesing and don't actually plot, you shouldn't have to import libGLU. This was brought up in Be able to import only VTK classes that do not need OpenGL #3283 and this has even led to people not using PyVista because we import the kitchen sink
• Improve import speed.
• Reduced risk to potential upstream vulnerabilities.

As such, I'd suggest that we rethink our imports and actually get away from using _vtk and simply import directly from vtkmodules. This was only really implemented to support backwards compatibility between vtk v8 and v9 and we can now get rid of this technical debt.

[MatthewFlamm]

I'm +1 on overall intent and am fine for this limited implementation. Possibly future looking comment: if we are going forward with lazy importing everything, or importing at point of use, then we should think about helper functions that avoid having this boilerplate for each reader.

[codecov]

Codecov Report

Merging #4337 (2a3bed3) into main (b9ee9b9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

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##             main    #4337   +/-   ##
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  Coverage   95.78%   95.78%           
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  Files          97       97           
  Lines       20773    20800   +27     
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+ Hits        19897    19924   +27     
  Misses        876      876           

[MatthewFlamm]

One small comment, otherwise LGTM