Add base velocity tracking task

[davidegorbani]

This PR adds a task to the IK to track a reference base velocity.

CC @Gianlucamilani

[traversaro]

If it is easy to do, could it make sense to generalize this as "link" or "frame" velocity tracking task? Or it is not as simple as I imagine?

[S-Dafarra]

If it is easy to do, could it make sense to generalize this as "link" or "frame" velocity tracking task? Or it is not as simple as I imagine?

I would agree with this. I understand that the difference is that the base velocity is usually a decision variable and thus it is quite useful as a regularization. At the same time, we don't have other "base" related methods. One possibility could be to have a "if" to use the trivial implementation in case the link is the base, with the proper sizes of the corresponding matrices

[davidegorbani]

Actually, I just realized that maybe this task is not necessary, as setting the kp_linear to zero in the R3 task would have the same result as a task to track the velocity of a given frame.