[Fix] CircularResistiveWall wake function sign

Fix approximated longitudinal wake function sign in CircularResistiveWall

collective_effects/resistive_wall.py

@@ -147,7 +147,7 @@ class CircularResistiveWall(WakeField):
                                       np.cos( np.sqrt(3) * t / self.t0 )  
                                       - np.sqrt(2) / np.pi * val )
         else:
-            wl = (-1/(4*np.pi * self.radius) 
+            wl = (1/(4*np.pi * self.radius) 
                   * np.sqrt(Z0 * self.rho / (c * np.pi) ) 
                   / time**(3/2) ) * self.length
             ind = np.isnan(wl)