Enforce an underlying structure of the skeletons about assistive-rehab

[pattacini]

@vvasco I've just made a few quick tests and the optimization is working nicely.
Thereby, the optimization is now on by default since 4cc949b.

[pattacini]

From 02be274 the code to optimize skeletons limbs is available. To enable this feature, turn true the flag optimize-limblength of the skeletonRetriever.

The implementation of the constant Hessian is still missing.

Finally, given that as it is openPose can provide only keypoints and segments and cannot discriminate any rotation of e.g. the shoulder yaw, it is not feasible at the moment to enforce the right sign among the limb angles.

[pattacini]

#39 finalized this development.

Original
Optimized