some bugfixes

particle.py

@@ -207,6 +207,7 @@ class Trajectories:
                 traj = Trajectories(part[slice_part],unraveled=self.unraveled)
             else:
                 traj += Trajectories(part[slice_part])
+        traj.frame_velocity = self.frame_velocity
         return traj
     def add_particles(self,part):
         import numpy
@@ -306,7 +307,7 @@ class Trajectories:
             lastPeriod = thisPeriod
         # Restore previous ravel state
         if restore_ravel_state and not was_unraveled:
-            self.ravel()
+            self.enforce_periodicity()
         return out
     def slice_time(self,time_beg=None,time_end=None):
         return
@@ -321,7 +322,7 @@ class Trajectories:
     def unravel(self):
         import numpy
         if self.unraveled: return
-        raise NotImplementedError('Implemented but untested!')
+        #raise NotImplementedError('Implemented but untested!')
         self._make_data_array()
         for axis in range(0,3):
             if self.period[axis] is not None: