[Fix] CircularResistiveWall longitudinal wake function

The approximated longitudinal wake function in CircularResistiveWall correctly takes into account the length

collective_effects/resistive_wall.py

@@ -151,7 +151,7 @@ class CircularResistiveWall(WakeField):
         else:
             wl = (-1/(4*np.pi * self.radius) 
                   * np.sqrt(Z0 * self.rho / (c * np.pi) ) 
-                  / time**(3/2) )
+                  / time**(3/2) ) * self.length
             ind = np.isnan(wl)
             wl[ind] = 0