Allow importing multiple solutions in inspector

[FHell]

It would be fantastic to allow loading several solutions, and switching between them, in the inspector.

[hexaeder]

Switching between them is possible using the set_sol! methods.

What I'd like to have additionally is something like a two-solution option, to compare them in the same window (like one "ref" solution always dotted in the background). If you have concrete interface suggestions I am happy to hear them!

[FHell]

Eventually it would be great to have a drop down menu in the interface to switch between different solutions but that's not super important.

For comparing trajectories - good question. We can brainstorm something on the blackboard.