old_contents: def compute_coordinate_transformations(piv):
      pass
  
new_contents: def compute_coordinate_transformations(piv, width, height, dt):
      ''''
      Compute Coordinate system  transformation from
            ------ y              y
           |                      |
           |             to       |
           x                       ------ x
  
      with x  ->  -y , u -> -v , y -> x, v -> u
      and time scaling
      '''
      pass
  
  
subject: Add postprocess doctstring.