Make theory covmat ordering automatically match ordering of datasets in runcard

Created by: RosalynLP

Right now we insist that people write datasets in the runcard in the same order as procs_index (i.e. when they're grouped by process). This is because at the fit stage the theory covmat is added to the exp covmat as a matrix so it must be aligned with the order of exps in the runcard.

I need to make it so that after generating the theory covmat, the covmat is reindexed so the order matches the runcard order.