code: def _surfdens(self,R,z,phi=0.,t=0.):
          
          return (3.-self.alpha)/2./nu.pi*z*R**-self.alpha\
              *special.hyp2f1(0.5,self.alpha/2.,1.5,-(z/R)**2)
docs: NAME:
             _surfdens
          PURPOSE:
             evaluate the surface density for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             the surface density
          HISTORY:
             2018-08-19 - Written - Bovy (UofT)