Add unit="1" to quaternions.

[HansOlsson]

Note that Modelica.Mechanics.MultiBody.Frames.Quaternions.der_Orientation already have a corresponding unit. That has two implications:

• It is the correct unit; as it cannot be anything else. (Also clear from the constraint that q*q=1.)
• In many cases we could already deduce the unit for the quaternion based on its derivative.

However, if we didn't compute the derivative in a specific model that deduction wouldn't happen. Additionally it seems a bit counter-intuitive to go that indirect route.