When requested mpi threading level cannot be provided, some sort of error should be reported

[ClaytonGDavis@…]

When mpi::environment requests a threading level with one of the constructors:

explicit environment(threading::level, bool = true);
environment(int &, char **&, threading::level, bool = true);

there is no guarantee that the requested threading level is actually available. Boost::mpi receives the actual threading level provided, but ignores it.

Instead, I suggest that an exception should be thrown when the provided level of threading support is less than requested. This would, for example, catch the common scenario where the MPI library was only compiled for single-threaded programs.