Introduced IMU 2.6

[pattacini]

As per request #570, this PR introduces IMU version 2.6.
Changes are regarded with:

• iKin to declare the new kinematics.
  Differently from what I planned, HN matrices are accumulated to adapt to the current code.
• iKinGazeCtrl to extend the set of allowed IMU's.

[Nicogene]

I tried it on iCubGenova02 using iKinGazeCtrl . Looking left and right the neck moves ok, the problem are the eyes that start do crazy movements.

Any idea what could be the problem?
It seems a sign problem somewhere 😅

[pattacini]

@Nicogene give the matrix HN a second look: I might have reported it wrongly.

[Nicogene]

Ok I will modify and test it

[Nicogene]

Probably also other part of the library (example) should manage this new head_version