The free-functions hyteg::applyDirichletBC() no longer depend on PETSc,
but were still in header files such as P2Petsc.hpp. This merge collects
them all into a new file sparseassembly/DirichletBCs.hpp.
This allows to remove the following, now empty, header files
- composites/petsc/P1StokesPetsc.hpp
- composites/petsc/P2P1TaylorHoodPetsc.hpp
- composites/petsc/P2P2StabilizedStokesPetsc.hpp
- p2functionspace/P2Petsc.hpp
and cleanup includes.