WIP: [test][1p] Add 3d convergence test

Adds a 2d test on a sphere tet grid (sphere.msh). I also committed a sphere_quad.msh grid which has some nasty distorted quad elements. Unfortunately I currently get for both grids:

Dune::InvalidStateException [decompose:/Users/pumbaa/dune-master/dumux/dumux/discretization/cellcentered/wmpfa/facedatahandle.hh:101]: CoNormal decomposition not found

although the tet version of the grid looks fairly nice.

I wanted to use this test to check if it works in 3D.

For now it's only Dirichlet but it should be relatively easy to try Neumann boundaries.