xyst

Fix [Cho,Loh]CG advection stabilization

* Multiplying with edge length was wrong, this commit removes it from
  all fluxes. In ChoCG this affects stabilization, in LohCG this affects
  artificial viscosity.

* Makes stabilization optional in ChoCG, see 'stab' in control file, on
  by default.

* Remove ChoCG Taylor-Galerkin flux, pressure is inconsistent at
  half-step.