Bugfix and additional rotation angle output.

This is a combination of a bugfix for the 2D rotation in the GPlates plugin and the improvement of the model output to visualize the models easily in ParaView (provide the rotation angles to rotate the model in its proper alignment). 
Please do not merge right now, since I am still checking the results for bugs. I just wanted to post this, because I have a license question. Since VTK uses a quite special formalism for the rotation of a model, I was not motivated to derive the necessary equations myself and code them up (all available sources I had did not derive this kind of rotation angles). To save time I just copied the code from ParaView and plugged it in to the function paraview_angles_from_matrix(). The Copyright notice of the according file is listed at: http://www.kitware.com/Copyright.htm . From this it seems we are allowed to use the code, but would need to mention the authors, maybe in the documentation of the function? I could put the following Header in the documentation of the function, which is what is stated in their .cxx file:

  Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
  All rights reserved.
  See http://www.kitware.com/Copyright.htm for details.

     This software is distributed WITHOUT ANY WARRANTY; without even
     the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
     PURPOSE.  See the above copyright notice for more information.

Maybe I could also skip the second part, since it is stated in the GNU license of Aspect anyway? Any ideas?
:-)