[Solvers] Move HQPOutput to a dedicated file

CMakeLists.txt

...n/formulations/inverse-dynamics-formulation-acc-force.hpp

...invdyn/formulations/inverse-dynamics-formulation-base.hpp

include/pininvdyn/solvers/fwd.hpp

include/pininvdyn/solvers/solver-HQP-base.hpp

include/pininvdyn/solvers/solver-HQP-eiquadprog-fast.h

include/pininvdyn/solvers/solver-HQP-eiquadprog-rt.hpp

include/pininvdyn/solvers/solver-HQP-eiquadprog-rt.hxx

include/pininvdyn/solvers/solver-HQP-eiquadprog.hpp

include/pininvdyn/solvers/solver-HQP-output.hpp

src/formulations/inverse-dynamics-formulation-acc-force.cpp

src/solvers/solver-HQP-base.cpp

src/solvers/solver-HQP-eiquadprog-fast.cpp

src/solvers/solver-HQP-eiquadprog.cpp

unittest/hqp_solvers.cpp

unittest/invdyn-formulation.cpp