Revert "Move the resolution of the optimization problem to the object defining it"

src/CMakeLists.txt

src/ZMPRefTrajectoryGeneration/ZMPVelocityReferencedQP.cpp

src/ZMPRefTrajectoryGeneration/ZMPVelocityReferencedQP.h

src/ZMPRefTrajectoryGeneration/problem-vel-ref.cpp

src/ZMPRefTrajectoryGeneration/problem-vel-ref.hh