circular lag function
[y]=crlag(x,n)
* x = input vector (tx1) or (1xt)
* n = # of values to return (optional; default = t)
* y = a (nx1) (if x is (1xt)) or a (1xn) (if x is (1xt))vector with:
- y(1) = x(n)
- y(2) = x(1)
...
-y(n) = x(n-1)
y=crlag([1:5]) // returns: y=[5 1 2 3 4]