Commit 219dd9 defaulted the linear solver to a sequential one. Since
the box Richards test is running in parallel, a parallel linear
solver has to be set. The AMGBackend gives slightly different results
above the fuzzy threshold. Therefore, also the reference solution is
adapted.

Argument dependent lookup deduces the Dune namespace from
the function arguments that are also in namespace Dune.

Because config.h is not in the same directory, but added via
the include path by -I.

For binary compatibility, Dune only uses the C bindings
of MPI. MPI_FOUND is deprecated from CMake but Dune does
defines it and encourage its use.
(reviewed by timok)

Fix all warnings triggered with -Wshadow.

See FS#250 for details.