`CoNLLDocument.document_table` is changed in `CoNLLDocument.get_string_representation`

[MPvHarmelen]

Because CoNLLDocument.document_table is a (mutable) list of lists, line 531 also changes the original table, because it is not copied in the following line:

cort/cort/core/documents.py

Line 528 in c637594

new_table = self.document_table