Remove all items. index names are removed if ``clear_indices==True``.

def clear(self, clear_indices=False):
        'Remove all items. index names are removed if ``clear_indices==True``.'
        super(MIMapping, self).clear()
        if clear_indices:
            self.indices.clear()
        else:
            for index_d in self.indices[1:]:
                index_d.clear()