Various improvements related to CavityResonator

Add update_feedback method to CavityResonator to force FB update when needed (when QL or tuning is changed).
The feedback reference targets (setpoints) for ProportionalIntegralLoop is now CavityResonator Vc and theta as for ProportionalLoop.
Add an init_FFconst method to ProportionalIntegralLoop which is called at initialization only (not any more after each setpoint change) and needs to be called manually if we want to change the feedfoward constant during tracking.
Some simplifications on the expressions in ProportionalIntegralLoop.
CavityMonitor now records Vc and theta.
Add option "mean" to plot_cavitydata to plot the mean phasor voltage and angle versus time for non empty bunches (now the default behaviour).
Add optinal parameter show_objective to cavitydata, if True shows the Cavity voltage and phase objective value in plots.