Create entity for SE3 trajectory generation

[andreadelprete]

We currently have a trajectory generator dedicated to the joints, and another one for trajectories in Euclidian space. We lack a trajectory generator for SE3, which we will need for the foot trajectories.

[andreadelprete]

I've created a first version of the SE3 trajectory generator in this commit 943a909 (even though the orientation part is not working yet)