Write experiments_index without experiment.load()

Created by: Zaharid

We waste a ridiculous amount of time in loading an experiment when it gets big, because of the inefficient way in which the covmat is constructed (#25 (closed)). This currently slows down vp-setupfit for no good reason at all. We should rewrite it in such a way that it doesn't have to load the experiment. We could as well use #412.