ENH: Remove warning: ‘itk::DiffusionTensor3DRead<float>’ declared with g...

...reater visibility than the type of its field ‘itk::DiffusionTensor3DRead<float>::m_Reader’

See comments: https://github.com/Slicer/Slicer/pull/121
Due to the fact that itkDiffusionTensor3DRead was declaring a private variable of type itkImageFileReader<DiffusionImageType>::Pointer and that this class was declared with ITK_ABI_EXPORT, we had to add ITK_ABI_EXPORT to the declaration of itkDiffusionTensor3DRead to remove the previously described warning when ResampleDTIVolume was compiled (shared object).