code: def _evaluate(self,R,z,phi=0.,t=0.):
          
          r2= R**2.+z**2.
          rb= nu.sqrt(r2+self.b2)
          return -1./(self.b+rb)
docs: NAME:
             _evaluate
          PURPOSE:
             evaluate the potential at R,z
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             Phi(R,z)
          HISTORY:
             2013-09-08 - Written - Bovy (IAS)