Fix bertscore references
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I added some type checking for metrics. There was an issue where a metric could interpret a string a a list. A `ValueError` is raised if a string is given instead of a list.

Moreover I added support for both strings and lists of strings for `references` in `bertscore`, as it is the case in the original code.

Both ways work:
```
import nlp

scorer = nlp.load_metric("bertscore")
with open("pred.txt") as p, open("ref.txt") as g:
    for lp, lg in zip(p, g):
        scorer.add(lp, [lg])
score = scorer.compute(lang="en")
```

```
import nlp

scorer = nlp.load_metric("bertscore")
with open("pred.txt") as p, open("ref.txt") as g:
    for lp, lg in zip(p, g):
        scorer.add(lp, lg)
score = scorer.compute(lang="en")
```

This should fix #295 and #238 
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