Update planetary_system.py

Author: rieder

examples/textbook/planetary_system.py

@@ -102,7 +102,7 @@ def main(Mstar, Ndisk, fmdisk, Rmin, Rmax, t_end, n_steps):
     write_set_to_file(star_and_planets, filename, 'amuse',
                       attribute_names=planet_attributes,
                       overwrite_file=True)
-    write_set_to_file(disk, filename, 'amuse', attribute_names=disk_attributes, overwrite_file=True)
+    write_set_to_file(disk, filename, 'amuse', attribute_names=disk_attributes, append_to_file=True)
 
     gravity_hydro = bridge.Bridge(use_threading=False)
     gravity_hydro.add_system(gravity, (hydro,) )
@@ -135,7 +135,7 @@ def main(Mstar, Ndisk, fmdisk, Rmin, Rmax, t_end, n_steps):
                           attribute_names=planet_attributes,
                           overwrite_file=True)
         write_set_to_file(disk, filename, 'amuse',
-                          attribute_names=disk_attributes, overwrite_file=True)
+                          attribute_names=disk_attributes, append_to_file=True)
 
         Ekin = gravity_hydro.kinetic_energy 
         Epot = gravity_hydro.potential_energy