2D solutions in 3D tests
I stumbled across a series of tests where 3D meshes were tested with only 2-dimensional solutions e.g. https://i10git.cs.fau.de/hyteg/hyteg/-/blob/master/tests/hyteg/convergence/P1ElementwiseCGConvergenceTest.cpp . This doesn't matter in hindsight as the solvers are used in practice on 3D problems and work. I will add 3D solutions in these tests.