Draft: Implement the Tree Edit Distance metric

Closes #3

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assigned to @starride

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• 1e30ec1a - Implement TED accuracy

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• 4ce96e61 - 1 commit from branch master
• a9eec061 - Implement TED accuracy
• 1eacabce - Fix typos

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• 1fcd8a10 - Write doc and tests

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• 4214322d - Add zss as requirement

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• 18476beb - Add nltk as requirement

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• faa8c053 - Lint

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The implementation of this metric follows the code from Donut.

Since it's not really order-independent (trees are ordered by values before evaluation), we should discuss whether to keep this metric here or move it to the atr-ner-eval library.