ENH: Added under-relaxation to jump conditions

Updates to the jump boundary conditions

• provide an optional minimum jump value

    plane
    {
        type            fan;
        patchType       cyclic;
        jump            uniform 0;
        value           uniform 0;
        uniformJump     false;

        jumpTable       ( (1000 0) (-200 1) (20 2) (-0.75 3) );

        // Optional minimum jump value
        minJump         0;
    }

• add possibility to under relax the jump value. Currently only applied to the fanFvPatchField where it can be useful for steady, non-uniform jump set-ups to avoid instabilities/checker-boarding, e.g.

    plane
    {
        type            fan;
        patchType       cyclic;
        jump            uniform 0;
        value           uniform 0;
        uniformJump     false;

        // Optional under-relaxation
        relax           0.2;

        ...
    }

Current (old) behaviour

New behaviour with under-relaxation