Could MPI comm world feature remove need for PStream patch

[edwardsmith999]

In the help for icoFOAM (and others) there is the option:
-world <name> Name of the local world for parallel communication

As discussed, this allows one processor parallel cases and avoids assuming MPI_COMM_WORLD, see e.g.

https://develop.openfoam.com/Development/openfoam/-/merge_requests/398/diffs?commit_id=89f2cda3abe9010d3994802080950a64af92a5ba

                if (Pstream::nProcs() == 1)
                {
                    WarningInFunction
                        << "Running parallel on single processor. This only"
                        << " makes sense for multi-world simulation" << endl;
                    dictNProcs = 1;
                }

however, this also means the parallel single case will not write to processor0, see e.g.

    case_ = globalCase_/("processor" + Foam::name(Pstream::myProcNo()));
    if (Pstream::nProcs() > 1)
    {
        case_ = globalCase_/("processor" + Foam::name(Pstream::myProcNo()));
    }
    else
    {
        case_ = globalCase_;
    }