Show chips that were already labelled at the start of the epoch after unlabeled chips

[manthey]

We had contemplated altering the certainty score to do this, but after consideration think it would be better to not alter the certainty scores and instead just move chips labelled in previous epochs to the end of the list of chips to label. If appropriate, we could add a flag per superpixel to indicate if it had been labelled during the epoch.