Swap a force field atom id for another user-defined value.

        This modified all values in :attr:`MolecularSystem.system['atom_ids']`
        that match criteria.

        This function can be used to decipher a whole forcefield if an
        appropriate dictionary is passed to the function.

        Example
        -------
        In this example all atom ids 'he' will be exchanged to 'H'.

        .. code-block:: python

            pywindow.molecular.MolecularSystem.swap_atom_keys({'he': 'H'})

        Parameters
        ----------
        swap_dict: :class:`dict`
            A dictionary containg force field atom ids (keys) to be swapped
            with corresponding values (keys' arguments).

        dict_key: :class:`str`
            A key in :attr:`MolecularSystem.system` dictionary to perform the
            atom keys swapping operation on. (default='atom_ids')

        Returns
        -------
        None : :class:`NoneType`

def swap_atom_keys(self, swap_dict, dict_key='atom_ids'):
        """
        Swap a force field atom id for another user-defined value.

        This modified all values in :attr:`MolecularSystem.system['atom_ids']`
        that match criteria.

        This function can be used to decipher a whole forcefield if an
        appropriate dictionary is passed to the function.

        Example
        -------
        In this example all atom ids 'he' will be exchanged to 'H'.

        .. code-block:: python

            pywindow.molecular.MolecularSystem.swap_atom_keys({'he': 'H'})

        Parameters
        ----------
        swap_dict: :class:`dict`
            A dictionary containg force field atom ids (keys) to be swapped
            with corresponding values (keys' arguments).

        dict_key: :class:`str`
            A key in :attr:`MolecularSystem.system` dictionary to perform the
            atom keys swapping operation on. (default='atom_ids')

        Returns
        -------
        None : :class:`NoneType`

        """
        # Similar situation to the one from decipher_atom_keys function.
        if 'atom_ids' not in self.system.keys():
            dict_key = 'elements'
        for atom_key in range(len(self.system[dict_key])):
            for key in swap_dict.keys():
                if self.system[dict_key][atom_key] == key:
                    self.system[dict_key][atom_key] = swap_dict[key]