Order columns based on nonzero count.
p = COLPERM(S) generate a permutation that reorders the
columns of the sparse (or full) matrix S in nondecreasing
order of nonzero count. Useful as a preordering for LU
Not the best ordering in the world, but it's fast to
compute, it does a pretty good job, and it shows that
we, too, can write tricky one-liners.
See also COLMMD27BOBX, SYMMMD , SYMRCM .