- There is an RAII wrapper called ScopedInitializer to handle
  finalization due to C++ exceptions and to avoid calls to Finalize in
  all exit branches.
- Fixed signature of MpiCommunicator::Init to match that of MPI_Init.

It should not be called "base" because inheritance does not work for the
constexpr methods.

Since CUDA 9 nvcc seems to be able to handle the templates behind Mesh.