Fix usage of OpenMP reduction in waLBerla macro in free-surface MaxVelocityComputer

This merge requests updates the free-surface LBM extension's MaxVelocityComputer, where a waLBerla macro is used, which contains an OpenMP reduction of a Vector3. However, it is not possible to reduce individual components of a Vector3 using the bracket-syntax. Therefore, this merge request splits the Vector3 into three separate variables.