A dynamicMotionSolverFvMesh must now use a "motionSolver" or
"motionSolvers" entry to select the underlying motion solver. For
example, in constant/dynamicMeshDict:
dynamicFvMesh dynamicMotionSolverFvMesh;
motionSolverLibs ("librigidBodyMeshMotion.so");
motionSolver rigidBodyMotion;
...
Previously the motion solver could also be specified with the keyword
"solver", but this resulted in a name clash with rigid body solvers
which are frequently specified in the same scope. For this reason, the
"solver" and "solvers" entries have been removed.