Add helper bones for humans

[ClemensLinnhoff]

Is your feature request related to a problem? Please describe.
Helper bones help to position the 3D model of a human with respect to another object, e.g. a car seat.

Describe the solution you'd like
Add helper bones to human 3D model structure.

[LudwigFriedmann]

It's a feature request. I'll include helper bones in the human structure (#38)

[ClemensLinnhoff]

It's a feature request. I'll include helper bones in the human structure (#38)

I would propose to to it in a separate PR. I would like to merge the human structure branch fast, because I would like to check if the structure works in general before adding more to it.