Add methods to compute longitudinal Twiss and invariant

Add Synchrotron.get_longitudinal_twiss method to compute the longitudinal Twiss parameters and the synchrotron tune for single or multi-harmonic RF systems.
Change back synchrotron_tune to provide only the usual the (unperturbed) synchrotron tune from main RF voltage.
The longitudinal invariant Js is now computed in Bunch.cs_invariant and Beam.bunch_cs and save by BeamMontior and BunchMonitor, its value is 0 by default if the longitudinal Twiss parameters are not provided in the Synchrotron class.