flowRateInletVelocity: Added option to extrapolate the velocity profile to the inlet

e.g.

    inlet
    {
        type                flowRateInletVelocity;
        massFlowRate        0.2;
        extrapolateProfile  yes;
        rho                 rho;
        rhoInlet            1.0;
        value               uniform (0 0 0);
    }