[fix] Staggered velocity output

It seems that in the lambda function the wrong fvGeometry has been used. It seems that this actually doesn't make any difference in the solution because the lambda is always called with the fvGeometry passed to the calculateVelocityForStaggeredGrid_.

Maybe we could also think about deleting the fvGeometry argument when calling getFaceVelocity. I current don't really see why one would need a different fvGeometry.