Rerwrite delta95z cut to handle new P(z) normalization

[deapplegate]

The P(z) being passed in is now defined as a density function, ie int P(z)dz = 1. Current delta95z cut does not handle that correctly.

At line https://github.com/deapplegate/Clusters/blob/master/pzmassfitter/maxlike_masses.py#L144