Feature/APE-advection

This MR ports the advection operator in the APE class from EquationSystem::WeakDGAdvection to AdvectionWeakDG and removes the continuous galerkin code.

As far as i can see, none of the existing advection classes would work for Continuous+APE. It also never had boundary conditions implemented, so i doubt anybody ever used it. Therefore i think removing it is our best choice here.

I tested this branch on my local machine and on buildbot. Everything looks fine.