Swap among models / metadata covariates

[agoldst]

Just putting a marker here for a significant rewrite for the next version of dfr-browser, which allows for swapping among models or metadata covariates. (Visualizing more than one model / covariate at once is a yet later revision). I have been sitting on working code for this for a while, but I don't want to release it until the companion data-export functions for dfrtopics are ready.