New joint controller: cascaded position/velocity combo

[PeterBowman]

As a follow-up to #262, I'd like to add another PID controller between the newly added position PID and the embedded current controller. The goal here is to achieve a stiff behavior, since a position-to-torque loop essentially makes it behave like a torsion spring (following the Hooke's law). The embedded implementation (inside the iPOS firmware) is, in fact, a cascaded 3-level PID (position-velocity-current).

See also:

• P091.055.MCII.STP.UM.pdf