keep_only_tokens should be renamed to keep_only_ner_tokens

In https://gitlab.com/teklia/atr/dan/-/blob/66f0f92db68d23c1312c617c8556169b5423e18c/dan/manager/metrics.py#L254, function keep_only_tokens should be renamed to keep_only_ner_tokens (and all occurences of this function). Same for keep_all_but_tokens that should be renamed to keep_all_but_ner_tokens.