Class that implements the Isochrone potential
NAME:
__init__
PURPOSE:
initialize an isochrone potential
INPUT:
amp= amplitude to be applied to the potential (default: 1)
b= scale radius of the isochrone potential
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:
2013-09-08 - Written - Bovy (IAS)