Add stero_3d function to mlab

Along the lines of the move/pitch/yaw functions I just submitted, one other function would be of great use for my use same of stereoscopic visualization: an mlab function that sets the stereoscopic 3d mode.  While it's pretty straightforward once you know where to look, when I first started exploring, it took me hours to figure out how to enable these modes.  This function packages it up quite neatly in an easy-to-apply mlab interface.