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.

[jeremy-murphy]

@aminiussi , if you still use this library, it is in desperate need of help to fix existing infrastructure problems, so I am calling on all users to please help if they can.

[aminiussi]

@jeremy-murphy I do not use graph_parallel (and never did if I remember correctly). I think graph parallel uses Boost.mpi, thus the dependency.
I can help if something need to be done in Boost mpi to make graph_parallel life easy.