OptionalncpNoncentrality parameter. If omitted, the central distribution is used.
As in R, an explicit ncp: 0 describes the same distribution as the central one. R then uses
the noncentral algorithm, so its values can differ slightly from the central values;
rStats.js returns values of the same accuracy in both cases.
Noncentrality option accepted by the chi-squared, t, F and beta distribution functions.