Adding metrics

To add a new metric, add a function in metrics.py using the same layout as the function coco. The type of input is important, and MUTT currently supports exporting corruptions in json and xml. After adding the function to metrics.py, add a metrics_file, a line calling the function, and a close statement in the section of mutt.py that has the comment "Run metrics tests", similar to how coco, terp, and badger are set up.

Current metrics

• CIDEr
• Bleu
• ROUGE
• METEOR
• BADGER
• TERp