Add gravity vector target in HumanStateProvider

[lrapetti]

This PR adds what was discussed in https://github.com/robotology/human-dynamics-estimation/issues/277, by:

• implementing the interfaces for selecting a single component of the rotation in the DynamicalInverseKinematics (plus the possibility of selecting also the position components
• on the HumanStateProvider I have created a new target type called gravity which select only the z component of the rotation as in the formula in https://github.com/robotology/human-dynamics-estimation/issues/277#issue-1093247974
• in the visualizer, I have add the possibility to visualize such targets by visualizing the gravity vector in the current frame (i.e. $Rz^{meas}$

[lrapetti]

This PR is done onto development branch add-different-target-types, so I will proceed merging, and then from that branch we will open the PR towards master.