code: def _evaluate(self,R,z,phi=0.,t=0.):
          
          if not self.isNonAxi:
              phi= 0.
          x,y,z= bovy_coords.cyl_to_rect(R,phi,z)
          if self._aligned:
              return self._evaluate_xyz(x,y,z)
          else:
              xyzp= numpy.dot(self._rot,numpy.array([x,y,z]))
              return self._evaluate_xyz(xyzp[0],xyzp[1],xyzp[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:
             2016-05-30 - Started - Bovy (UofT)