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.

tracking/monitors/__init__.py

@@ -10,7 +10,6 @@ from mbtrack2.tracking.monitors.monitors import (Monitor, BunchMonitor,
                                                  BeamMonitor,
                                                  ProfileMonitor,
                                                  WakePotentialMonitor,
-                                                 TuneMonitor,
                                                  CavityMonitor,
                                                  BunchSpectrumMonitor,
                                                  BeamSpectrumMonitor)
@@ -21,4 +20,6 @@ from mbtrack2.tracking.monitors.plotting import (plot_bunchdata,
                                                  plot_wakedata,
                                                  plot_tunedata,
                                                  plot_cavitydata,
-                                                 streak_beamdata)
+                                                 streak_beamdata)
\ No newline at end of file
+
+from mbtrack2.tracking.monitors.tools import merge_files
\ No newline at end of file