Calculating correlation edits the input metrics

[danieldeutsch]

During the correlation calculation, multiple metrics objects are merged into one https://github.com/danieldeutsch/sacrerouge/blob/f1a864cd8b89542466f50315b249e532c3d0ca9a/sacrerouge/commands/correlate.py#L34-L38 If someone calls correlate multiple times in a row, it will mess up the results. This should not happen. We should instead make a copy of all of the objects so we don't change the input objects.