set seed=None in sim() by default and reset it to int(time.time()) in…

… function's body, as default arg in python func is fixed when func is defined

BxB.py

@@ -46,12 +46,14 @@ def sim(n_points = 5000, n_turns = [1000, 1000, 0, 5000], kick_model = "precise"
         n_sigma_cut = 5,
         density_and_field_interpolators_n_cells_along_grid_side = [500, 500],
         n_random_points_to_check_interpolation = 10000,
-        select_one_turn_out_of = 1000, seed = int(time.time()), output_dir = "tmp",
+        select_one_turn_out_of = 1000, seed = None, output_dir = "tmp",
         output = "integrals_per_turn avr_xy_per_turn integrals_per_particle avr_xy_per_particle points"):
   '''
   Helper function returning "Sim" structure with the parameters of the simulation to be used
   in the "beam_beam()" function. Not specified parameters are filled with the default values.
   '''
+  if seed is None:
+    seed = int(time.time())
   return Sim(n_points = n_points, n_turns = n_turns, kick_model = kick_model,
              n_sigma_cut = n_sigma_cut,
              density_and_field_interpolators_n_cells_along_grid_side =