New LongRangeResistiveWall class to deal with multi-bunch and multi-turn wakes from resistive wall.
Add Mpi.share_means method to share bunch means using MPI.

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.

Beam.mpi_share_distributions is now moved to mpi.share_distribution
Now mpi4py is only needed for the multi-bunch tracking and not for single bunch

Add tracking to CavityResonator based on the mbtrack implementation but using a analytic formula for speed-up
Add method to Beam to share longitudinal distributions
Add methods to Mpi to look for next and previous bunches
Add start of a CavityMonitor, to improve to deal with multi-cores

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