code: def _evaluate(self,R,z,phi=0.,t=0.):
          
          return -1./nu.sqrt(R**2.+(self._a+nu.sqrt(z**2.+self._b2))**2.)
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:
             2010-07-09 - Started - Bovy (NYU)