Rename BPFile* engine to BP3File* engine

Created by: pnorbert

BP4 engine is coming, so let's keep "BPFile" as a name for the default engine (BP3 now, BP4 later) without any change for the users.

Then IO.cpp should look like this when opening an engine:

    if (isDefaultEngine || engineTypeLC == "bpfile")
    {
        if (mode == Mode::Read)
        {
            engine = std::make_shared<engine::BP3FileReader>(*this, name, mode,
                                                            mpiComm);
            m_EngineType = "bp3file";
        }
        else
        {
            engine = std::make_shared<engine::BP3FileWriter>(*this, name, mode,
                                                            mpiComm);
            m_EngineType = "bp3file";
        }
    }
    else if (engineTypeLC == "bp3" || "bp3file")
    {
        //  BP3FileReader, BP3FileWriter
    }
    else if (engineTypeLC == "bp4" || "bp4file")
    {
        //  BP4FileReader, BP4FileWriter
    }

Later we can change to BP4 engine when BPFile with Read mode is selected, when BP4 can read both old and new data format. And finally, we can change to BP4 engine when BPFile with Write mode is selected, when we decide BP4 is ready to be the default engine.