enable Gouraud shading (or Phong, either is fine) with pyvista for meshes in example notebooks

Pretty pictures ftw! Itk vs pyvista shown. Let's make pyvista prettier. Instructions here: https://docs.pyvista.org/examples/02-plot/shading.html

Underlying this issue is that rendering in the notebooks has taken a significant turn for the worse, becoming so slow that we removed it from the notebooks and now use a separate window. It may be that upgrading to vtk 9 (#725) will fix things.