Explore feasibility of simplifying vtk symbol imports

[johnkit]

Needs more definition, but the underlying idea is to copy what Vedo does for importing objects. Most of the logic is in https://github.com/marcomusy/vedo/blob/master/vedo/vtkclasses.py. The main features of interest

• internal module lookup for each class
• class name aliasing (e.g. "vtk" prefix not required)