Fluctuate fits the model which has a single population which has been growing
(or shrinking) according to an exponential growth law. It estimates 4Nu and
g, where N is the effective population size, u is the neutral mutation rate
per site, and g is the growth rate of the population.
Fluctuate forms part of the Lamarc (Likelihood Analysis with Metropolis
Algorithm using Random Coalescence) suite. See:
http://evolution.genetics.washington.edu/lamarc.html
