Added new Python API for connecting pipelines

A new (Python-only) API to connect and execute pipelines was introduced. Pipeline objects can now be connected with the >> operator. For example, the following

s = vtkSphereSource()
sr = vtkShrinkFilter()
sr.SetInputConnection(s.GetOutputPort())

can now be expressed as

s = vtkSphereSource()
sr = vtkShrinkFilter()
s >> sr

It is also possible to do the following:

pipeline = vtkSphereSource() >> vtkShrinkFilter()

Pipelines can also be executed more easily. For example:

result = (vtkSphereSource() >> vtkShrinkFilter()).update().output

We also introduced an API to use filters in a more functional way:

result = vtkShrinkFilter(shrink_factor=0.5)(input_data)