Fix free space CSR expression

Take into account length

collective_effects/csr.py

@@ -80,7 +80,7 @@ class FreeSpaceCSR(WakeField):
         Beams 7.5 (2004): 054403.
         """
         
-        Zl = (self.Z0 / (2*np.pi) * gamma(2/3) * 
+        Zl = (self.Z0 * self.length / (2*np.pi) * gamma(2/3) * 
               (-1j * 2 * np.pi * frequency / (3 * c * self.radius**2) )**(1/3) )
         return Zl