Change back invariant name to cs_invariant
Simplify SkewQuadrupole track method

Remove for-loops inside the function.

- cs_invariant method in Bunch is renamed to 'action'.
- Action is now saved together with BunchMonitor and can be plotted in plot_bunchdata

Main change
- Variable local_beta, local_alpha, and local_gamma are made property objects.

Minor change
- Add an option to plot phase space in different style.

Change track method to allow for Beam objects
If pynaff can not find the tune, the tune value is nan.
Nanmean and nanstd are used to avoid to bias the tune value.
! Untested if some particles are lost !

WakePotentialMonitor now use interpolation on a fixed grid to be able to save the wake potential data (which size change from turn to turn).

Correct scale in plot_wakedata
Avoid circular import by moving import in function

- Option to plot bunch current on x-axis has been included in plot_bunchdata.
- Option to select the plot size for plot_phasespacedata has been added.

- Shift of x-axis when start is changed has been fixed.
- Added option to plot other dimensions apart from 'tau'.
- Improved code conciseness and efficiency.

A new function for plotting profile monitor has been added. Two types of plot can be obtained : bunch profile evolution and streak plot.

Plotting routines now return figure object.

Two new plotting routines have been added
- plot_optics for plotting optical variables (beta, alpha, gamma, despersion)
- plot_beamdata for plotting data recorded by BeamMonitor

Rebase test_plot on master
plot_phasespacedata -> only "particles" dataset is plotted
plot_phasespacedata -> mandatory arguments

- total_size and save_every input parameters have been removed.
- only_alive option function has been changed
- Some small corrections on scaling of delta, bunch_charge, bunch_current

- "plot_bunchdata" and "plot_phasespacedata" have been separated from particles module.
- Input prompt has been removed from the code.
- Plotting method for variables in Beam object such as "bunch_current" and "bunch_charge" with respect to bunch number has been added.
- Only alive particles plotting option has been added.

- total_size and save_every input parameters have been removed.
- only_alive option function has been changed
- Some small corrections on scaling of delta, bunch_charge, bunch_current

- "plot_bunchdata" and "plot_phasespacedata" have been separated from particles module.
- Input prompt has been removed from the code.
- Plotting method for variables in Beam object such as "bunch_current" and "bunch_charge" with respect to bunch number has been added.
- Only alive particles plotting option has been added.