getPi: Calculate the mixture proportions from the zero model / spike mass model
residuals.
Description
F(x) = 1 / (1 + exp(-(x-m)/s)) (the CDF of the logistic distribution).
Provides the probability that a real-valued random variable X with a given
probability distribution will be found at a value less than or equal to x.
The output are the mixture proportions for the samples given the residuals
from the zero model.