Renaming particle directions

[AntonReinhard]

We have the type ParticleDirection with subtypes Incoming and Outgoing. This makes sense for the view outside of a process, where the particles either "go into" or "come out of" the process. However, I also like to use these directional types "inside" the calculations of Feynman diagrams, where there is no set outside or inside, so the names become arbitrary.

Maybe it would make sense to rename these to a ParticleSign instead, and have a Positive and Negative or similar. It then becomes somewhat intuitive inside a diagram, especially with the conservation of momenta.

I'm not entirely sure about this though since the names may be more confusing for someone coming from outside the project and only caring about the process-level view. Maybe we could just add aliases for Incoming -> Positive and Outgoing -> Negative or the other way around?

[AntonReinhard]

I just noticed this issue should probably go into QEDbase since that is where the particle directions are defined. We can discuss here first anyways I think.