code: def _surfdens(self,R,z,phi=0.,t=0.):
          
          r= numpy.sqrt(R**2.+z**2.)
          Rma= numpy.sqrt(R**2.-self.a**2.+0j)
          if Rma == 0.:
              za2= (z/self.a)**2
              return self.a*(2.+numpy.sqrt(za2+1.)*(za2-2.))/6./numpy.pi/z**3
          else:
              return (self.a*Rma**-3\
                          *(numpy.arctan(self.a*z/r/Rma)-numpy.arctan(z/Rma))
                      +z/(r+self.a)/(R**2.-self.a**2.)).real/2./numpy.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)