Add neural coreference resolution system

Add the neural coreference resolution system by Fynn Schröder and Hans Ole Hatzel (https://github.com/uhh-lt/neural-coref, paper).

The associated merge request

• adds the model code of the upstream repository to the codebase,
• add neural_coref implementation for the coref component.

Remaining TODOs:

• How to handle the incomplete output of the neural coref model? In particular, the coref system does not define a 'main' mention, hence, e.g., "coref_resolved" is undefined.
• Upload model files to the MONAPipe-Data dataverse