NRandPerm Random permutation.
NRandPerm(n,nret) is a random permutation of the integers from 1 to n,
of which nret elements are returned.
NRandPerm thus returns a random selection of integers from 1 to n with
For example, NRandPerm(6,4) might be [2 4 5 6].
Note that NRandPerm calls RAND and therefore changes RAND’s state.
See also PERMUTE, RANDPERM.