Returns the quantile (inverse CDF) for the given probability p.
Computed as xm * (1-p)^(-1/α). Returns scale for p=0 and +∞ for p=1.
xm * (1-p)^(-1/α)
the value x at which cdf(x) = p.
cdf(x) = p
the cumulative probability, must be in [0, 1].
[0, 1]