Added a new module, vtkPythonInterpreter.

VTK now includes a Utilities module, vtkPythonInterpreter that provides a Python
interpreter. While its use it totally optional, it helps in applications with
embedded Python interpreter, such as ParaView.

vtkMatplotlibMathTextUtilities now uses vtkPythonInterpreter to initialize
Python. This enables applications to setup Python environment as needed witout
directly having to hack environment variables to setup matplotlib environment.

Since vtkPythonInterpreter allows applications to initialize/finalize Python
interpreter mutliple times, fixed vtkPythonCommand to cleanup
PyObject/PyThreadState pointers when Python interpreter is exitted.

Change-Id: I80ed56fc3f64dda2766a6aa2e6e92c3aa7290993