Use extract geometry filter to enforce PolyData

[banesullivan]

VTKjs currently only supports vtkPolyData. These changes use an extract geometry filter to get a useable representation of any VTK data object.

Note that these changes make creating compelling scenes with vtki a breeze!!

Examples

import vtki
vtki.set_plot_theme('document')
from vtki import examples
import panel
panel.extension('vtk')
from panel.pane import VTK as panel_display

def display(p):
    return panel_display(p.ren_win)

data = examples.load_uniform()

p = vtki.Plotter(notebook=False, off_screen=True)

p.add_mesh(data.threshold_percent([0.25, 0.75]))
p.add_mesh(data.wireframe())
p.add_mesh(data.slice_orthogonal())

p.show(auto_close=False)
display(p)

data = examples.download_st_helens().warp_by_scalar()

p = vtki.Plotter(notebook=False, off_screen=True)
p.add_mesh(data, opacity='linear')
p.show(auto_close=False)
display(p)

data = examples.download_lidar()

p = vtki.Plotter(notebook=False, off_screen=True)
p.add_mesh(data)
p.show(auto_close=False)
display(p)

[banesullivan]

Note these changes will make pyvista/pyvista#144 possible:

[philippjfr]

Looks good to me, I've restarted the Windows build which timed out the first time. Will merge shortly.