@simonpf This branch shows what I was talking about during the meeting.

The new files contain the structs for more limited return types I was after, and also the full implementation of the general scatterer for tro spectral.

It also wraps the visiting functions in scattering_species.cc in a way that you don't have to implement all of the interface for every type. E.g., a type that cannot be TRO does not need the TRO interfaces.

Had to make the mutex change as part of this branch because there was a kernel panic somewhere (only in CI, I cannot see it locally, so it might be a very narrow problem)