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

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

ring.L instead of ring.L0