dynamicMultiMotionSolverFvMesh discussion

1. only provision is for motion on cellZone, not on cellSet
2. cellZone specification might easily be inconsistent with sub-motion solver (e.g. solidBodyMotionSolver) which looks for a cellZone or cellSet specification itself. (as long as this selection includes the multimotion cell selection it is ok)
3. output of solidBodyMotion might be confusing: e.g. applying solid body motion to entire mesh which then gets truncated by the dynamicMultiMotionSolver