Expect either boost::optional or std::optional from Boost.MPI

[aminiussi]

Hi,

Boost.MPI is moving from boost::optional to std::optional when the later is available (unless explicitly asked otherwise).
During the transition phase, clients can expect either, which implies to

• only rely on the common API between boost::optional and std::optional
• take the Boost MPI type from boost::mpi::optional

I'll submit a pull request with those modifications.

[aminiussi]

The pull request #40 implement that change.