Creates an internal list where the variables with dimensionality larger than one are expanded.
        This list is the one that is used internally to do the optimization.

def _expand_space(self):
        """
        Creates an internal list where the variables with dimensionality larger than one are expanded.
        This list is the one that is used internally to do the optimization.
        """

        ## --- Expand the config space
        self._expand_config_space()

        ## --- Expand the space
        self.space_expanded = []
        for variable in self.space:
            self.space_expanded += variable.expand()