code: def _evaluate(self,R,z,phi=0.,t=0.):
          
          return self._mn3[0](R,z,phi=phi,t=t)\
              +self._mn3[1](R,z,phi=phi,t=t)\
              +self._mn3[2](R,z,phi=phi,t=t)
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:
             2015-02-07 - Written - Bovy (IAS)