Wakefield : Bugfix on ComplexData addition + Addition on Wakefields + Improve ImpedanceModel

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