code: def _evaluate(self,R,z,phi=0.,t=0.):
          
          r= numpy.sqrt(R**2.+z**2.)
          return -numpy.log(1.+r/self.a)/r
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)