Output of MadgwickAHRS

[gabrielebndn]

This is not a priority and might be not needed if we drop Madgwick altogether as planned.
The output of MadgwickAHRS is a 4-element vector representin a quaternion, in the order [w,x,y,z].
This is not coherent with other entities we are using, (for instance, all those in src/matrix/operator.cpp), forcing useless reordering steps in the middle.