l distributions.

Measures the agreement between two normal probability distributions.
Returns a value between 0.0 and 1.0 giving the overlapping area in
the two underlying probability density functions.

    >>> N1 = NormalDist(2.4, 1.6)
    >>> N2 = NormalDist(3.2, 2.0)
    >>> N1.overlap(N2)
    0.8035050657330205
z$Expected another NormalDist instancez(overlap() not defined when sigma is zerorµ