Add non_zero option for steady command

preprocessor@9db21a23 introduced non_zero option for displaying only non-zero residual equations. We should allow the same option for steady, which calls

    if ~options_.noprint
        if ~isempty(oo_.steady_state)
            display_static_residuals(M_, options_, oo_);

Thus, if you have 200 equations overall and only 1 with residuals, steady will display all equations, while resid(non_zero) will only display one.