Transform EMC DCC dataset to spaCy format

[lcreteig]

N.B. The code addressing this issue should not be added to this repo, but to https://github.com/umcu/emc-dcc-transformation instead

For the context module from medspacy to recognise negated entities, we first need to transform the ECC dataset to Doc objects that can be fed to a (med)spaCy pipeline:

• [x] Store the texts from the .txt. files as a spaCy doc. Probably serialise to spaCy DocBins pickled docs?
• [x] Add the labeled terms from the .ann files as spaCy entities.