ENH: Added visualization of transforms

Features:
* Support non-linear transforms in the Transforms module: allow Apply, Harden, Invert transform
* Transform information displayed in the Transforms module (type of transform, basic properties)
* Transform visualization:
** Built into the Transforms module
** Three main modes: Glyphs (show an array of arrows, cones, spheres), Grid (show a deformed grid), or Contour (show isolines/isosurfaces for specified displacement magnitude values)
** All transform types are supported (chains of transforms as well)
** Visualization in the slice viewers
** Visualization in the 3D viewers, in the specified region (region can be a slice viewer, a volume, or a ROI widget)
** Real-time update: if the transform (or any visualization parameter) is changed then the visualization is updated immediately (interactive visualization while editing the transform)
** Built-in colormap editor
* MetaImage (mha), NIFTI (nii) vector volumes can be loaded as displacement field (grid) transform