@classmethod
def load_system(cls, dict_, system_id='system'):
    """
        Create a :class:`MolecularSystem` from a python :class:`dict`.

        As the loaded :class:`MolecularSystem` is storred as a :class:`dict` in
        the :class:`MolecularSystem.system` it can also be loaded directly from
        a :class:`dict` input. This feature is used by :mod:`trajectory` that
        extracts trajectory frames as dictionaries and returns them
        as :class:`MolecularSystem` objects through this classmethod.

        Parameters
        ----------
        dict_ : :class:`dict`
           A python dictionary.

        system_id : :class:`str` or :class:'int', optional
           Inherited or user defined system id. (default='system')

        Returns
        -------
        :class:`pywindow.molecular.MolecularSystem`
            :class:`MolecularSystem`

        """
    obj = cls()
    obj.system = dict_
    obj.system_id = system_id
    return obj
