Add variables and constraints for L1 norm minimization.

def _add_minimization_vars(self):
        """Add variables and constraints for L1 norm minimization."""

        self._z = self._prob.namespace(self._model.reactions, lower=0)

        # Define constraints
        v = self._v.set(self._model.reactions)
        z = self._z.set(self._model.reactions)

        self._prob.add_linear_constraints(z >= v, v >= -z)