Misc monitor changes

Remove TuneMonitor (remplaced by BunchSpectrumMonitor).
Remove default values for save_very, buffer_size, total_size.
Set default value for mpi_mode to False.
Add "xtau"/"ytau" choices for Bunch/BeamSpectrumMonitor classes.
Set default value for n_fft to None to avoid zero-padding by default.
Fix WakePotentialMontior n_bin (x2) problem.