Add forces visualization in HumanStateVisualizer

[lrapetti]

Since I was working on force visualization with the iDynTree based visualizer, I was thinking of adding it also to the HumanStateVisualizer so that we get closer to the Rviz based visualizer aspect.

[lrapetti]

The implementation can be found in feature/add-arrow-viz. The feature is already working:

arrow_visualization_trim.mov

the only point before opening the PR and merging is that it depends on a fix done by @S-Dafarra (see robotology/idyntree#836 (comment)) and not yet merged in master.
As soon as it is merged I will proceed with this feature.