'Return the most similar users to the given user, ordered
from most similar to least.
Parameters
user_id:  int or string
ID of user for which to find most similar other users
how_many: int
Desired number of most similar users to find'

def most_similar_users(self, user_id, how_many=None):

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