add containment similarity metric

[ulf1]

J(A, B) = | INTERSECTION(A,B) | / | UNION(A,B) |
C(A,B) = | INTERSECTION(A,B) | / | A | 

Assume A=Q is a shingleset that we want to query against a database full of shinglesets x_i (i.e. B=x_i for specific database example i).

Literature:

• page 2, https://arxiv.org/pdf/1809.00458.pdf
• docs at http://ekzhu.com/datasketch/lshensemble.html

[ulf1]

def containment(Q: set, X: set) -> float:
    u = float(len(Q.intersection(X)))
    return u / len(Q)

[ulf1]

why?