'Return the candidate items that could possibly be recommended to the user
Parameters
user_id:  int or string
ID of user for which to find most similar other users
data_model: The data model that will be the source for the possible
candidates'

def candidate_items(self, user_id, data_model, **params):

    raise NotImplementedError('BaseCandidateItemsStrategy    is    an    abstract    class.')
