Add warning when metrics scores overwrite each other.

[yoavkatz]

Right now , metrics score overwrite each other (Not referring to the 'score' and 'score_name' , but two metrics can have the same 'f1' score.

Users find it confusing. We should at first step make this into a warning.

"Metric 'token_overlap' has the same score as another metric in the metrics list ['token_overlap', 'macro_f1'] . To avoid overwriting the value , add a score_prefix to the metric (e.g. score_prefix="token_overlap_") . For more information, see ... "