Misc improvements on wakefield.py

Add effective impedance calculation to ImpedanceModel
Rework plot_area to support transverse impedance and add better legend and scaling
double_sided_impedance : new function to add negative frequencies to impedance spectrums
effective_impedance : tuneS as input, use double_sided_impedance and xi input is used
beam_loss_factor : uses double_sided_impedance
Correct a few bugs in wakefield.py