message: ENH: get_system accepts calsses with Atoms base calss
diff: @@ -53,11 +53,11 @@ class Descriptor(with_metaclass(ABCMeta)):
  :class:`.System`: The given system transformed into a corresponding
  System-object.
  """
  - system_type = type(system)
  - if system_type == Atoms:
  - return System.from_atoms(system)
  - elif system_type == System:
  + if isinstance(system, Atoms):
  + if type(system) == System:
  return system
  + else:
  + return System.from_atoms(system)
  else:
  raise ValueError(
  "Invalid system with type: '{}'.".format(system_type)
  