Add the possibility to add/remove targets after the initialization

[lrapetti]

Now targets in inversevelocitykinematics and dynamicalinversekinematics (PR #265) can be added/removed only before running the first initialization and can not be changed in real-time.

We should implement this feature.

[lrapetti]

Now there is the possibility to activate or de-activate targets.

If not active, however, the error is simply set to zero (see here), however this implies that the target is setting velocity to zero (which is not the same thing). So this still needs to be fixed by adding a way to remove them (or at least to add a weight for the targets in the QP, which can be set to zero).