Update stress balance command-line options. See #144.

All tests pass. Now I need to update all the scripts and document new
the new option.

-stress_balance
 {none,prescribed_sliding,sia,ssa,prescribed_sliding+sia,ssa+sia}

Old options that are affected: -no_sia, -sia (not used, I bet),
-ssa_sliding, -ssb_constant (used once).
Update -ssa_sliding option in examples/*.