Add a new subkey to this key.

        Parameters
        ----------
        subkey : AdfKey
            A new subkey.

        Notes
        -----
        Duplicate check will not be performed if this is an 'Atoms' block.

def add_subkey(self, subkey):
        """
        Add a new subkey to this key.

        Parameters
        ----------
        subkey : AdfKey
            A new subkey.

        Notes
        -----
        Duplicate check will not be performed if this is an 'Atoms' block.

        """
        if self.key.lower() == 'atoms' or not self.has_subkey(subkey):
            self.subkeys.append(subkey)