MPI tutorial sample has missing includes

[Ryo IGARASHI <rigarash@…>]

There are a lot of sample codes in Boost.MPI tutorial documentation. It is very useful, but there are some missing include:

In the point-to-point sample and the non-blocking sample, it lacks <string> header.

In the gather sample, it lacks <vector> header include.

In the reduce sample 2, it uses std::plus<> so that #include <functional> is needed.

[Ryo IGARASHI <rigarash@…>]

diff file for mpi.qbk

[Matthias Troyer]

(In [55417]) Fixed ticket 2581