Allow for particle losses in Monitor classes

Add a is_empty property to the Bunch class.
Add a check_empty optional parameter to the Monitor.track_bunch_data method.
ProfileMonitor: if the bunch is empty, no profile is saved.
WakePotentialMonitor: if the bunch is empty, the same from last good turn is saved.
BunchSpectrumMonitor: if the bunch is empty, the incoherent spectrum is nan.