Computed test_output and display_output by generalizing intrinsic.

disp_output now takes a typ argument (default: 'display') that
specifies how to map the values to outputs.  Four options are
 * 'intrinsic': takes diff (val - ref)
 * 'extrinsic': takes rel error (val - ref) / ref
 * 'ratio': (val / ref)
 * 'display': just (val)