randperms

A modified version of randperm, users have more choice to the returned value.

Description:
randperms(n) is a random permutation of the integers from 1 to n.
randperms(m,n) when m > n, returns the nth element of randperm(m);
when m < n, returns the permutation of the intergers
from m to n.
randperms(m,n,idx) returns the idx th element of permutated intergers
from m to n.