ENH: freestream BC: allow inlet value to be specified by another patch

The freestreamFvPatchField previously employed a fixed value when supplying
the inlet values.  This commit extends the BC so that users can use another
patch to supply the values via the new freestreamBC entry, e.g. to set the
velocity to an atmospheric boundary layer profile:

    inlet
    {
        type            freestream;
        freestreamBC
        {
            type            atmBoundaryLayerInletVelocity;
            flowDir         (1 0 0);
            zDir            (0 0 1);
            Uref            20;
            Zref            20;
            z0              uniform 0.1;
            zGround         uniform 935;
        }
    }

The earlier specification is also maintained for backwards compatibility, e.g.

    inlet
    {
        type            freestream;
        freestreamValue uniform (300 0 0);
    }