Refactors StokesPCGSolverOld to become dimension independent

Commit changes StokesPCGSolverOld so that it will work in 2D and 3D.
By using the new VectorFunctions and Div/DivT operators the solver
becomes agnostic implementationwise w.r.t. dimension.

Commit also enhances the P2P1StokesSchurCGConvergenceTest by adding a
test case for 3D. Convergence not fast so we currently just make five
iterations to see that it works technically.

Also renames P2P1StokesSchurCGConvergenceTest. New name is closer to
the class name: P2P1StokesPCGSolverConvergenceTest
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