Rotation

[tberlok]

This PR implements two new features in Paicos:

• GPU support for visualization routines
• An orientation class which allows for rotating the view in slices and projections (both on CPU and GPU)
• A change in the definition of an image in the 'y'-direction.

People who want the old 'y' direction behaviour can recover this by passing an orientation object to instead of a direction:

orientation = pa.Orientation(normal_vector=[0, -1, 0], perp_vector1=[1, 0, 0])