Revert "Change name of class"

src/Mathematics/FootConstraintsAsLinearSystemForVelRef.cpp

src/Mathematics/FootConstraintsAsLinearSystemForVelRef.h

...ectoryGeneration/OnlineStepPositionTrajectoryGeneration.h

src/ZMPRefTrajectoryGeneration/ZMPVelocityReferencedQP.cpp

src/ZMPRefTrajectoryGeneration/ZMPVelocityReferencedQP.h

src/ZMPRefTrajectoryGeneration/generator-vel-ref.cpp

src/ZMPRefTrajectoryGeneration/generator-vel-ref.hh