IMU calibration

[mgrouch]

To have the right Heading I have to enter an offset of 30.
When I do that pitch and roll do not work well :
rolling the IMU make now moving both roll and pitch data!
It make no sense for me because the offset of the heading should not affect roll and pitch.

Offset of the heading changes the axis relative to which you measure pitch and roll. 
So pitch and roll would change if you modify offset. Only at level zeros would match.