Change how empty bunches are dealed with

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

Tracking/beam.py

@@ -23,6 +23,7 @@ class Bunch:
         Bunch current in [A]
     alive : bool, optional
         If False, the bunch is defined as empty 
+        
     Attributes
     ----------
     mp_number : int
@@ -249,7 +250,10 @@ class Beam:
         
     Methods
     ------
-    init_beam()
+    init_beam(filling_pattern, current_per_bunch=1e-3, mp_per_bunch=1e3)
+        Initialize beam with a given filling pattern and marco-particle number 
+        per bunch. Then initialize the different bunches with a 6D gaussian
+        phase space.
     """
     
     def __init__(self, ring, bunch_list=None):