Class that implements the Hernquist potential
NAME:
__init__
PURPOSE:
Initialize a Hernquist potential
INPUT:
amp - amplitude to be applied to the potential
a - “scale” (in terms of Ro)
normalize - if True, normalize such that vc(1.,0.)=1., or, if given as a number, such that the force is this fraction of the force necessary to make vc(1.,0.)=1.
OUTPUT:
(none)
HISTORY:
2010-07-09 - Written - Bovy (NYU)