Use quaternions instead of rotation matrixes as much as possible

[1-Bart-1]

The advantages would be:

• Quaternions are more computationally efficient
• Quaternions don't have singularities
• The code gets better structured and therefore easier to understand: instead of having quaternions + rotation matrixes + euler angles we only have quaternions for computation and euler angles for interpretability

A disadvantage might be that quaternions are tricky to understand. But with good functions / documentation this should be fine.

[ufechner7]

Well, there are not many conversions that are really required. The main conversion function is calc_heading(). Feel free to write a version of this function that uses quaternions. But it is not time critical, because it is called only once per timestep and not in the callback of the DAE solver.