Fisher probability distribution
[pdf]=fdis_pdf(x,a,b)
* x = a vector
* a = numerator dof
* b = denominator dof
* pdf = a vector of pdf at each element of x of the F(a,b) distribution
x=fdis_pdf([2.1 3.4 5.1],2,52) // Returns the vector of probabilities that a Fisher(2,52) is greater than 2.1, 3.4 and 5.1 respectively.