Transforming aligned volumes with different rotations

[drouin-simon]

Two volumes with embeded rotations are sometimes transformed differently when their common parent is transformed. This could be caused by the fact that origin and step of image objects are stored in the vtkImageData while the rotation read from a file is stored in the object's transform, which leads to an incorrect order in the transforms.

The solution that should be considered is to store the origin and step with the rotation in the transform of the volume and always have origin = (0,0,0) and step = (1,1,1).