Ordering algorithms. Currently, these are all local.

Natural: The identity permutation, mostly for testing.
Random:  Random permutation, mostly for testing.
RCM:     Reverse Cuthill-McKee for bandwidth reduction.
SortedDegree: Sorted (Increasing) vertex degree, poor man's version of AMD.
AMD:     Approximate Minimum Degree, requires TPL.

TODO: Scotch, Spectral.
