code: def _evaluate(self,R,phi=0.,t=0.):
          
          return 0.5*R*R*(1.+2./3.*R*numpy.sin(3.*phi))
docs: NAME:
             _evaluate
          PURPOSE:
             evaluate the potential at R,phi,t
          INPUT:
             R - Galactocentric cylindrical radius
             phi - azimuth
             t - time
          OUTPUT:
             Phi(R,phi,t)
          HISTORY:
             2017-10-16 - Written - Bovy (UofT)