Calculating correlation edits the input metrics

[danieldeutsch]

During the correlation calculation, multiple metrics objects are merged into one

sacrerouge/sacrerouge/commands/correlate.py

Lines 34 to 38 in f1a864c

	for metrics in metrics_list:
	if metrics.summarizer_id not in metrics_dicts[metrics.instance_id]:
	metrics_dicts[metrics.instance_id][metrics.summarizer_id] = metrics
	else:
	metrics_dicts[metrics.instance_id][metrics.summarizer_id].merge(metrics)

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.