This function returns the value of the i-th element of the
permutation p. If i lies outside the allowed range of 0 to
n-1 then the error handler is invoked and 0 is returned.
— Function: int gsl_permutation_swap (gsl_permutation * p, const size_t i, const size_t j)
This function exchanges the i-th and j-th elements of the
permutation p.