Joint impedance ctrl interface

This solution works in simulation. I have tested the PositionControlDirect. To test the impedance I have set the stiffness to 0.0 so the robot collapse. I have not tested yet the Impedance in the real robot to check if the same Interfaces are used. Max/Min Impedance values are read from ini files, if not specified some default values are used. PD values are loaded also from ini files using the already implemented function to get PIDs. Let me know. 

IMPORTANT: now the stiffness is specified as Nm/rad, maybe we need to change it in Nm/grad to be more consisten? (ecco che succede a non essere SI)