[All] Move robot-wrappers to robots and inv-dyn-formulations to formulations folders

CMakeLists.txt

include/pininvdyn/contacts/contact-base.hpp

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

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

include/pininvdyn/robots/robot-wrapper.hpp

include/pininvdyn/tasks/task-base.hpp

src/CMakeLists.txt

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

src/formulations/inverse-dynamics-formulation-base.cpp

src/robots/robot-wrapper.cpp

src/tasks/task-actuation-bounds.cpp

src/tasks/task-actuation-equality.cpp

unittest/invdyn-formulation.cpp

unittest/robot-wrapper.cpp