Add missing MPI function instantiations.

Add missing MPI function instantiations.

The fixed-length array (i.e., things declared like T(&values)[N])
versions of MPI::sum, MPI::max, etc, all live in the header file mpi.h
since the length (N) is a compile-time constant. Those functions all
call Utilities::MPI::internal::all_reduce, which is declared in
mpi.templates.h, so that function (all_reduce) must be present in the
compiled library.

Note for the curious: it turns out that most compilers and most compiler
settings will not inline this function, meaning that it consistently
shows up in the shared object files without this explicit instantiation
in debug mode and usually in release mode. In particular, GCC 6 does not
inline it (so this instantiation is not necessary) but clang 3.8 does (so
this instantiation is necessary). Some versions of GCC (5.3 but not 4.8)
seem to inline it with the '-march=native' flag.