test COM trajectory tracking in gazebo

mv nd_trajectory_generator from unitest to python/test as it is not unitary

remove joint_trajectory generator because nd_trajectory_generator is sufficient, simpler and more generic
add "create_entities_utils.py" to instanciate entities
add an unit test for nd_trajectory_generation controlling 1 joint in position in gazebo

entites are derived from sot-torque-control package

addition to previous commit