Add method to ProcessObject to convert ITK Image Types

SimpleITK does not support itk::Image's of Vector or FixedArray. The
buffers essentially contain the data, and can be considered just large
arrays for floats/double. This conversion method does a reinterpret
cast on the buffer.

Change-Id: I5c1a2bab0a991d67d03e232a3e4ea9b01f257ccd
Add public utility function to convert itk's VectorImage

Careful consideration was made to where to place the header with this
function. This is in the class of utility functions which require ITK,
since it's a template function it's just placed in the Common include
directory, so that it can easily be re-used.

Change-Id: I6ca5daa49c842445f3ea846255c152c96df3dcf4
Adding displacement field input image

pts = sitk.PhysicalPointSource(size=[100,100])
ptx = sitk.VectorIndexSelectionCast(pts, 0)/100 * 3.14 * 2
pty = sitk.VectorIndexSelectionCast(pts, 0)/100 * 3.14 * 2
displacement = sitk.Compose(sitk.Sin(ptx)*5*sitk.Sin(pty/2),
sitk.Sin(-pty)*5*sitk.Sin(ptx/2) )

Change-Id: I2b620dff0c6872ff70187ffdfcea0f53f82bb915
Add wrapping for InvertDisplacementFieldImageFilter

The feature of adding an initial inverse is not include nor is
selecting an interpolator.

Change-Id: I6d1d406c51f2cd07ea85477f2a5fcf95d5b946cd
Adding wrapping for InverseDisplacementFieldImageFilter

Change-Id: Ia314f77d4e0996c0c47b6f70d0bb13978dd4f880
Merge topic 'SIMPLEITK-537_AddDisplacementFieldSupport' into next

4e0b8887 Adding wrapping for InverseDisplacementFieldImageFilter
00e679fb Add wrapping for InvertDisplacementFieldImageFilter
7e43dc67 Adding displacement field input image
6a16c2e8 Add public utility function to convert itk's VectorImage
b0d67ff9 Add method to ProcessObject to convert ITK Image Types