For a random variable X whose values are distributed according
to this distribution, this method returns P(X ≤ x). In other words,
this method represents the (cumulative) distribution function, or
CDF, for this distribution.
x - the value at which the distribution function is evaluated.
the probability that a random variable with this
distribution takes a value less than or equal to x
MathException - if the cumulative probability can not be
computed due to convergence or other numerical errors.