SPGSM stabilization for incompressible Navier-Stokes, heat transfer, and Boussinesq

SPGSM is an acronym I pulled from the Gravemeier and Wall paper for VMS-type stabilization of variable-density low Mach Navier-Stokes. This is basically doing the VMS business, but dropping some of the terms so you're left with PSPG and SUPG looking terms. There's still a couple of mysteries, but I figured I'd make the PR anyway so folks can pull from this and play.

1. The VMS part of the Boussinesq term in the momentum equations does really weird things in the convective_cell example; changes the time scale of the solution, different flow features, etc. So I've commented it out until we can better understand what's going on.

2. I'd swear that the current sign of the Boussinesq PSPG term is *wrong* (this applies to Adjoint stabilization as well) . But if I flip it to the "correct" version, the convective_cell solution is just wrong (energy gets sapped out above the hot zone...). As is, it gives solutions that look reasonable. Really need to understand what's going on there. A good test might be to pull out the PSPG terms from all the different stabilizations and use Taylor-Hood and see what happens. Note that if you pull out Boussinesq stabilization all together, you get different flow characteristics than with (which isn't necessarily surprising since you may lose consistency of the residual in the stabilization terms).