Add support for intelligent switching between simplicial up-looking and supernodal right-looking factorizations.
It is widely known that very sparse matrices can be factored more efficiently using a simplicial up-looking factorization rather than a supernodal right-looking factorization (see, for example, the user guide for CholMod http://faculty.cse.tamu.edu/davis/suitesparse.html).
Such a switch should be performed after the initial reordering, but before the supernodal relaxations are computed. The scaffolding for the switch is essentially already in place.
But, catamari does not yet support a multithreaded version of simplicial factorizations, and so support would need to be added for this switch to make sense in the multithreaded regime.