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

def most_similar_items(self, item_id, how_many=None):

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