Fix invalid read in vtkNewickTreeReader

This defect crept in during my recent work to improve the
vtkNewickTreeReader's performance while reading large trees.

Change-Id: I1a575adeca304a3a0b94aa3a7448bf6327ca6aea
Fixed bug in vtkCachingInterpolatedVelocityField.

vtkCachingInterpolatedVelocityField was passing -1 to GetCell() the
first time around. Since GetCell() expects a valid cell id, this was
causing all kinds of issues. Also, it was not calling FindCell()
the right way - it was always passing -1 for the cell id causing it
to use the locator every time. Fixed both.

Change-Id: I8025666e046f4b39d7b43d5c3b71f69f763b20ef
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
Merge topic 'fix_Newick_valgrind_defect' into master

8c745664 Fix invalid read in vtkNewickTreeReader
Merge topic 'refactor_python_interpretor' into master

be75a19a Added a new module, vtkPythonInterpreter.
Merge topic 'vtkCachingInterpolatedVelocityField-bug-fix' into master

b64fb665 Fixed bug in vtkCachingInterpolatedVelocityField.