Possibility to remove unnecessary files after metrics have been calculated

[avaucher]

The scripts to calculate the metrics, rxn-prepare-context-metrics, rxn-prepare-forward-metrics, rxn-prepare-retro-metrics, copy and generate many files that are not needed anymore once the metrics have been calculated:

• tokenized / detokenized ground truth files
• tokenized / detokenized prediction files
• etc

Especially when evaluating a lot of models, this ends up in a lot of disk space. Adding an option to remove the files at the end of the script could be helpful.