message: Modified MBTR to use the more general extended system creator.
diff: @@ -900,7 +900,8 @@ class MBTR(Descriptor):
  # If needed, create the extended system
  if self.periodic:
  - ext_system, cell_indices = self.create_extended_system(system, radial_cutoff)
  + centers = system.get_positions()
  + ext_system, cell_indices = self.create_extended_system(system, centers, radial_cutoff)
  else:
  ext_system = system
  cell_indices = np.zeros((len(system), 3), dtype=int)
  @@ -1004,7 +1005,8 @@ class MBTR(Descriptor):
  # If needed, create the extended system
  if self.periodic:
  - ext_system, cell_indices = self.create_extended_system(system, radial_cutoff)
  + centers = system.get_positions()
  + ext_system, cell_indices = self.create_extended_system(system, centers, radial_cutoff)
  else:
  ext_system = system
  cell_indices = np.zeros((len(system), 3), dtype=int)
  