This class is aware of a number of enviroment variables that can be used to
configure and debug python initialization (all are optional):

- VTK_MATPLOTLIB_DEBUG: Enable verbose debugging output during initialization
  of the python environment.

- VTK_MATPLOTLIB_PYTHONINTERP: Path to the python interpreter. This will be
  passed to Py_SetProgramName prior to calling Py_Initialize.

- VTK_MATPLOTLIB_PYTHONHOME: See the Python documentation on the PYTHONHOME
  environment variable. This will be passed to Py_SetPythonHome prior to
  calling Py_Initialize.

- VTK_MATPLOTLIB_PYTHONPATH: A list of additional python module paths to be
  prepended to the sys.path object after initialization. Use ';' on windows and
  ':' on apple/linux to separate multiple paths.

Change-Id: I00613c3254750d26de350514a426fe24ce26f732