STY: forces: change from singular to plural naming convention

src/functionObjects/forces/forceCoeffs/forceCoeffs.C

src/functionObjects/forces/forces/forces.C

src/functionObjects/forces/forces/forces.H

...dBodyMeshMotion/rigidBodyMeshMotion/rigidBodyMeshMotion.C

...ion/rigidBodyMeshMotionSolver/rigidBodyMeshMotionSolver.C

...cement/sixDoFRigidBodyDisplacementPointPatchVectorField.C

...sixDoFRigidBodyMotionSolver/sixDoFRigidBodyMotionSolver.C