Adapt the output functions for multi line models

[ufechner7]

For models that use multiple lines and winches instead of a KCU the signatures of the output functions needs to be extended.

In particular:

• unstretched_length
• tether_length
• winch_force
• spring_forces

1. and 2. and 3. shall return an SVector

For the last function there are different options:

1. return a SMatrix
2. return an SVector of SVectors
3. add a parameter like line_no to select the line to look at and just return an SVector

[ufechner7]

@1-Bart-1 Which of these options would you prefer?

[1-Bart-1]

All the segments are numbered in the PointMassSystem. So I think it should be fine to use just a single SVector with all the spring forces. These can the indexed later using the point_mass_system.tethers[1].segments if you want to extract all the spring_forces from the first tether for example.

[1-Bart-1]

I just return a Vector now, and for the pos I return a Vector of Vectors. I don't see a reason to use an SVector. For performance, returning a Matrix for the pos would be best, as the getter function from MTK returns a Matrix.

[1-Bart-1]

Todo:

• Unit tests that show that the output functions for the SymbolicAWEModel return the same data structures as for the KPS4 kite.
• Documented as common kite model api.

[1-Bart-1]

I don't agree with the hard type here. <:AbstractVector and <:AbstractMatrix is more flexible.