adding __len__ to dst class

ess/TraceWin.py

@@ -31,6 +31,9 @@ class dst:
             self._data = numpy.zeros((self.Np, 6))
             self.mass = mass
 
+    def __len__(self):
+        return self.Np
+
     def append(self, x=0.0, xp=0.0, y=0.0, yp=0.0, E=0.0, phi=0.0):
         """
         Append one particle to the distribution