fix bug in itk.GetVnlMatrixFromArray python wrapping

[ncullen93]

vnlObject is supposed to be arr.. seems like a copy-paste error.

>>> import itk
>>> import numpy as np
>>> itk.GetVnlMatrixFromArray(np.ones((3,3))) # doesnt work

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-696-efd313fbd347> in <module>()
----> 1 itk.GetVnlMatrixFromArray(np.ones((3,3)))

/Users/ncullen/anaconda/lib/python3.6/site-packages/itkExtras.py in GetVnlMatrixFromArray(arr)
    341     """Get a vnl matrix from a Python array.
    342     """
--> 343     return _GetVnlObjectFromArray(arr, "GetVnlMatrixFromArray")
    344 
    345 def GetVnlMatrixViewFromArray(arr):

/Users/ncullen/anaconda/lib/python3.6/site-packages/itkExtras.py in _GetVnlObjectFromArray(arr, function)
    326     PixelType = _get_itk_pixelid(arr)
    327     templatedFunction = getattr(itk.PyVnl[PixelType], function)
--> 328     return templatedFunction(vnlObject)
    329 
    330 def GetVnlVectorFromArray(arr):

NameError: name 'vnlObject' is not defined