Function S2CellId.fromFacePosLevel
Returns a cell given its face \(range 0..5\), Hilbert curve position within that face \(an unsigned integer with S2CellId::POS_BITS bits\), and level \(range 0..MAX_LEVEL\). The given position will be modified to correspond to the Hilbert curve position at the center of the returned cell. This is a static function rather than a constructor in order to indicate what the arguments represent.