enable querying by class similarity

[slizb]

Build the functionality for a user to pass an array-like object of class similarities.
Will return image classes that approach the specified similarity structure.

Hypothetical example, n = 2:
similarity = .8 might return classes "cat" and "fox"
similarity = .2 might return classes "cat" and "tugboat"

[slizb]

this work was started with compute_similarity.py