SUPG stabilization and equal-order elements

[jcallaham]

After #134 introduced the BDF solver, the next big performance/stability improvement would probably be from adding some stabilization. This might also let us use linear velocity elements, which would significantly reduce the size of the linear systems.

Links:

• https://github.com/florianwechsung/alfi/blob/master/alfi/stabilisation.py
• https://github.com/david-kamensky/VarMINT/blob/master/VarMINT.py
• https://en.wikipedia.org/wiki/Streamline_upwind_Petrov%E2%80%93Galerkin_pressure-stabilizing_Petrov%E2%80%93Galerkin_formulation_for_incompressible_Navier%E2%80%93Stokes_equations