CHange plotting behavior of LLik and Log PPr to avoid zoom window

[mikegilchrist]

Sometimes we don't want the zoom window when plotting the PPr or LLik traces. The default is 'zoom = NULL' and then creates one from last 10%. If you set zoom = NA it won't create the window but it will throw an error.

Proposed fix: Have code suppress window if zoom = NA but without throwing an error.