Numerical stability of Draco3 example

[Stéphane Caron]

@ymontmarin I just tested the new linear holonomic task on the humanoid_draco3.py example, and it seems it has become less numerically stable:

humanoid_draco3_issue.mp4

(The first run uses the latest version from PyPI, the second run is the current master branch.)

I don't think the gains should be reduced for the new task formulation? Or maybe this is because we are now including the free flyer in the task, whereas it was just q_2 = -q_1 before?

[Stéphane Caron]

Fixed in #58