Fixed error on the computation of the beam size with dispersion in Synchrotron.sigma method.
Add import of plot_beamspectrum and streak_beamspectrum

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.

Fix mbi_threshold to have sigma in [s}
Add plot_critical_mass to instabilities (ions)
Add sigma(position) to synchrotron to compute the beam size at any ring location

Add ABI threshold calculation
Bugfix synchrotron_tune computation
Add zoom option to ImpedanceModel.plot_area

Add T1 = RF period to synchrotron object
Add binning and plot_profile to Bunch object // plot_profile to improve
Modify beam.filling_pattern to store the array instead of computing it every time
Add distance_between_bunches method to Beam object
Modify not_empty method of Beam to use filling_pattern

Optic functions as arrays
Uses PyAT lattice for import
Adapt other files for the change

Update machines/__init__.py, machines/soleil.py, tracking/__init__.py, tracking/aperture.py, tracking/element.py, tracking/optics.py, tracking/parallel.py, tracking/particles.py, tracking/rf.py, tracking/synchrotron.py files

To be PEP8 complient:
Lowercase folders
Extended docstrings
Rename and divide some files

Add synchrotron radiation in transverse plane
Fix a mistake in the transverse one turn map

New parallel module to handle Mpi class which is used in Beam

Test OK, to improve