Added Python view that enables plotting data with matplotlib

This view is compiled only if Python is enabled.

The Python view accepts a Python script that can be used to plot
data in a matplotlib figure. The script must contain two functions
described below:

- setup_data(view)

  The setup_data function supplies the current Python view as an
  argument. This is the native VTK object on which all methods that
  have been wrapped in Python may be called. The Python view supplies
  some methods that can be used to select which data arrays to
  transfer from the server side to the client side (all rendering is
  performed on the client). No arrays are selected by default.

- render(view, figure):

  The render function supplies the current Python view as the first
  argument and a matplotlib FigureCanvasAgg as the second argument.
  The render function is typically expected to create one or more
  subplots in the figure that will be used for rendering.

The functions setup_data and render may be defined in the script
(e.g., when defined through the GUI settings for the Python View) or
in the global Python interpreter (e.g. when defined in a script run by
pvpython). They are invoked when needed by the Python view when
rendering is needed.

Removed unused saveImage() method from various views.

Updated VTK to origin/master

Change-Id: I882fa8eeacc81e8120762bf1312f568523559b09
Merge topic 'PythonView' into next

797ae3a Added Python view that enables plotting data with matplotlib