NAME:
vx
PURPOSE:
return x velocity at time t
INPUT:
t - (optional) time at which to get the velocity
vo= (Object-wide default) physical scale for velocities to use to convert
use_physical= use to override Object-wide default for using a physical scale for output
OUTPUT:
vx(t)
HISTORY:
2010-11-30 - Written - Bovy (NYU)
