Velocity vector image Function

I have added a function to pynbody.plot.sph called velocity_image that will produce a regular SPH image with velocity vectors overlaid using matplotlib's quiver().

It will  generate a grid of velocity vectors using the smoothed velocity field, and then overlay it on an smoothed image, along with a key showing vector scale.  Colors, etc. can be specificed, and keyword arguments are passed through to image(), allowing you to do all the regular things you can with it.