Initial OPLSDA group selectors

[dchiquito]

The frontend of the OPLSDA initializes with empty group selectors when the page is refreshed. The backend arbitrarily chooses two groups to render when no groups are specified, so the plot is rendered with those two groups. This is confusing, as you must choose both groups before the plot rerenders.

The solution is to find a way to always select the same two groups on the frontend and the backend.