code: def _evaluate(self,R,phi=0.,t=0.):
          
          return self._A*math.exp(-(t-self._to)**2./2./self._sigma2)\
              /self._alpha*math.cos(self._alpha*math.log(R)
                                    -self._m*(phi-self._omegas*t-self._gamma))
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:
             2011-03-27 - Started - Bovy (NYU)