Fix TD calculation for transverse dipolar wakes due to sign change in the 31/08/2021 commit.

Check that the bunch is not empty before binning in Mpi.share_distributions. If it is then the beam filling pattern is updated and empty array are sent.
In CavityResonator.track, the tracking without mpi is modified to take into account the case with an empty bunch. Mpi case is fine as the beam filling pattern is updated in the call for Mpi.share_distributions.
In WakePotential.track, if the bunch is empty tracking does nothing.

Fix the sign of the dipolar wake potential to get the correct sign of the tune shift in tracking with respect to the impedance/wake function conventions used in mbtack2.

Remove part which was causing "damping".
We have to re-process the wake function at each track call and not to reuse the previously processed wake function

WakePotential -> add method to plot wake potential for a perfect gaussian bunch
utilities -> new function gaussian_bunch which gives the gaussian bunch profile
utilities -> read_IW2D now also reads wake function from IW2D
utilities -> new function read_IW2D_folder which reads the full results folder of IW2D into a WakeField element.
resistive_wall -> change the sign of the wake functions to agree with the code convention

Fix dipole_moment due to switch to numpy arrays for Bunch.binning

1st test, not complete

Now based on two time arrays!
The bunch profile is computed from its own time array and interpolated into the wake function time array.
The input wake functions must be uniformly sampled!
Reference computation now gives loss factor and kick factor.
Tested with Wlong and Wxdip/Wydip -> OK
Untested for Wxquad/Wyquad.

Modify the docstrings and add some precisions.

A method to compare energy loss from a wake potential to a loss factor.
Minor changes :
- sigma method in Ring object has been edited.
- add WakePotential in __init__ in tracking module.

Remove mean of tau array during wake function interpolation to avoid wrong interpolation if tau is not centered on zero

Add transverse wake potential to WakePotential and move class to dedicated file
Merge Resonator and TransverseResonator into Resonator with dedicated file
wakefunction_from_imp -> change trimming to allow for a float value
wakefunction_from_imp -> transverse impedance divided by 1j, to be checked with impedance convention but ok with benchmark
Wakefield -> add wake_components