change OU prior so that it is flat on the proportion of variance explained

[rmcminds]

ouPercent <- function(x, alpha) {exp(-2 * alpha * (1 - x)) * (1 - exp(-2 * alpha * x)) / (1 - exp(-2 * alpha))}

x is proportion of time from root

sample ouPercent on the simplex with a flat prior, then calculate alpha with the inverse of the above function

[rmcminds]

when x is 0.5