Remove inactive elements from the DOM

[JosephBond]

Currently, when an intermediate value is de-selected (by moving our mouse off of a cell in the output matrix), the intermediate matrix stays visible. We will need to add functionality that can remove an unwanted visualization from the DOM, which is not something we've experimented with before. We've floated the idea of having a small widget for intermediates which handles all child elements underneath it, toggling them on and off as required. There will be some thought here, since we don't want unnecessary flickering.