ImpedanceModel -> new method to compute beam and bunch energy loss
beam_loss_factor -> drop 0 from data only if it is there, avoid bug if it is not

Changes in loss_factor to use frequency instead of omega
Read_CST forces to have positive real part of longitudinal impedances
New functions : Gaussian_bunch_spectrum, beam_spectrum, beam_loss_factor
ImpedanceModel.plot_area in GHz

Bugfix on ComplexData addition : sort data before interpolation
Wakefield class : Add static method to add wakefields
ImpedanceModel class : add methods to handle more easily the wakefields. Maybe can be improved more by using a single Wakefield of each type ?
ImpedanceModel class : add plot_area and summary. summary to be improved more

Add a ImpedanceModel object which derives from Element and is used to sum up different Wakefield elements
Fix uppercase module in resistive_wall.py

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