code: def _surfdens(self,R,z,phi=0.,t=0.):
          
          r2= R**2.+z**2.
          rb= nu.sqrt(r2+self.b2)
          return self.b*((R*z)/r2-(self.b*R*z*(self.b**2+2.*R**2+z**2))
                         /((self.b**2+R**2)*r2*rb)
                         +nu.arctan(z/R)-nu.arctan(self.b*z/R/rb))/R**3/2./nu.pi
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)