Computes and plots
the Weibull density (pdf) and cumulative distribution (cdf).
The parameters (θ and δ)
must be positive. (Too small θ leads to unreliable results.)
The distribution's mean, standard deviation, median and mode are computed.
For θ = 1, the Weibull becomes an Exponential
with parameter μ = δ
(and, of course, also σ = δ).
Plots the 'pdf' and 'cdf'. | 
