Detect repeated entities

[ninakokalj]

This pull request introduces functionality to detect and handle repeated entities within the text processed by the entity extraction pipeline. I also moved a couple of functions to the helpers.py file to reduce redundancy and improve code reusability. I moved merge_entities and _filter_entities out of the MultiExtractor class, _get_spacy_fields and _set_spacy_fields out of both NERExtractor and PatternExtractor classes.

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Closes #8

[eriknovak]

PR handled in a separate fork.