Pyvista: `PlotterITK.show(True)` only works as the last command of a Jupyter cell.

[russelldj]

I've found that while the examples provide for plotting interactively in a Jupyter notebook work (very well!), slight modifications cause it to not plot anything. The MVE is a print command immediately following the call to show, which causes there to be no plot. I would have expected this to work similarly to the base Plotter where plots can be generated in the middle of a cell and potentially many per cell. If this is a known bug/limitation/feature, perhaps it could be more clearly documented.

---

To Reproduce

import pyvista as pv

# create a mesh and identify some scalars you wish to plot
mesh = pv.Sphere()
z = mesh.points[:, 2]

# Plot using the ITKplotter
pl = pv.PlotterITK()
pl.add_mesh(mesh, scalars=z, smooth_shading=True)
pl.show(True) # works

```py
pl = pv.PlotterITK()
pl.add_mesh(mesh, scalars=z, smooth_shading=True)
pl.show(True) # Doesn't work
print("Showed a mesh")

**Screenshots**
If applicable, add screenshots to help explain your problem.
![image](https://user-images.githubusercontent.com/29384527/97108448-fb81cc00-16a3-11eb-9187-202ac61b61a2.png)


-----

**System Information:**

```txt
--------------------------------------------------------------------------------
  Date: Sun Oct 25 09:25:44 2020 Eastern Daylight Time

                OS : Windows
            CPU(s) : 8
           Machine : AMD64
      Architecture : 64bit
       Environment : Jupyter
             Intel : GPU Vendor
Intel(R) HD Graphics 630 : GPU Renderer
4.5.0 - Build 22.20.16.4749 : GPU Version

  Python 3.7.7 (default, May  6 2020, 11:45:54) [MSC v.1916 64 bit (AMD64)]

           pyvista : 0.25.3
               vtk : 9.0.1
             numpy : 1.19.0
           imageio : 2.9.0
           appdirs : 1.4.4
            scooby : 0.5.6
            meshio : 4.0.16
        matplotlib : 3.3.0
           IPython : 7.16.1
          colorcet : 1.0.0
             scipy : 1.5.1
        itkwidgets : 0.32.0
              tqdm : 4.48.0
--------------------------------------------------------------------------------

This is using jupyterlab==2.2.9 and Microsoft Edge. I can try to test it on other browsers/OSs if there is interest.

[akaszynski]

This is a valid point! The arises from how we're returning a itkwidgets.widget_viewer.Viewer object, but we're not actively displaying it. By default, anything at the end of the cell that stands on its own and ipython/jupyterlab displays that. What I should have done when creating this module is simply called display from within the show method:

from IPython import display
import pyvista as pv
pl = pv.PlotterITK()
pl.add_mesh(pv.Sphere())
out = pl.show()
display.display_html(out)
print('weeeee')

As you can see here, it works in this context, and this necessitates an update to the PlotterITK class.

[akaszynski]

Fixed in 0.27.0