Improve bb impedance

We modify the way the cut-off frequency is used for the longitudinal impedance. The cut-off frequency assumed in the input files is add and then the shunt impedance is scaled by the cut-off frequency given as input to the model. In this way the shunt impedance of the broadband resonators is always consistent with cut-off frequency chosen by the user.

We also add the possibility to specify different shunt impedances in x and y, while same shunt impedances were assumed before.