In the past MPI / parallel features were disabled until the
proper flag was added. This patch changes the behavior to
enable MPI iff it is found.

* Autotools: --disable-parallel and --enable-parallel work as
  before but the documentation is a bit clumsy to minimize changes.
* CMake: USE_MPI gets removed, too. It was not used by the Dune
  core modules. The backwards compatible --disable-parallel remains.