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

Add Yokoya_elliptic function to compute Yokoya factors for an elliptic shape.
Add an option to divide by value in read_CST
Add docstrings to read_CST and read_IW2D

Circular and rectangular tapers
Correct module name in tools.py
Add kick factor calculation for quadrupolar wakes (similar to dipolar ones)

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