ZELDA candidate lists in EntityLinker

[alanakbik]

Entity Linking has been shown to profit from candidate lists that contain for a mention a list of possible disambiguation targets.

This PR integrates candidate lists as loss masking into the EntityLinker. It also integrates the ZELDA candidate lists for convenience.

Example for initializing the ZELDA candidate lists and retrieving candidates for a string.

generator = CandidateGenerator(candidates="zelda", lower_case=False)
print(generator.get_candidates("Pence"))

Example for initializing an EntityLinker with a candidate generator:

linker = EntityLinker(
    embeddings,
    label_dictionary=tag_dictionary,
    pooling_operation="first",
    candidates=CandidateGenerator("zelda") 
)