'Returns the users that were most influential in recommending a given item
to a given user. In most implementations, this method will return users
that prefers the recommended item and that are similar to the given user.
Parameters
user_id : int or string
ID of the user who was recommended the item
item_id: int or string
ID of item that was recommended
how_many: int
Maximum number of items to return.
Returns
The list of users ordered from most influential in recommended the given item to least'

def recommended_because(self, user_id, item_id, how_many, **params):

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