code: def _evaluate(self,R,z,phi=0.,t=0.):
          
          #Cylindrical distance
          Rdist = _cylR(R,phi,self._orb.R(t),self._orb.phi(t))
          #Evaluate potential
          return evaluatePotentials( self._pot, Rdist, self._orb.z(t)-z, use_physical=False)
docs: NAME:
             _evaluate
          PURPOSE:
             evaluate the potential at R,z, phi
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             Phi(R,z,phi)
          HISTORY:
             2011-04-10 - Started - Bovy (NYU)
             2018-10-18 - Updated for general object potential - James Lane (UofT)