Enumerate the Cartesian angular momentum triples up to l_max.
Parameters:
l_max (int) – Maximum total angular momentum l=lx+ly+lz.
Returns:
All (lx,ly,lz) triples, grouped contiguously by total l in
ascending order (the CACE convention); (0,0,0) comes first. The
length is (l_max+1)(l_max+2)(l_max+3)/6.
