ENH: relative velocity usage for porosity in MRF regions

- New optional keyword 'MRFRelativeVelocity' (default=true)
usable by the porosity models. For example, in 'constant/porosityProperties':

    porosity1
    {
        type            DarcyForchheimer;
        ...

        MRFRelativeVelocity    yes;
        ...
    }