Add bunch_X properties to Beam class
Add a generator over "not empty" bunches to Beam class
Modify Bunch class in order to have mean, std and emit as property and not method

Empty bunches are now not None any more but represented by Bunch class with 1 mp set to dead
Change init_beam to reflect the change

Empty bunches are now not None any more but represented by Bunch class with 1 mp set to dead
Change init_beam to reflect the change

Use pandas DF to hold data in Bunch class instead of numpy masked array -> better behaviour and allows to set to alive particles only