Fusion Extension implementation - inter-species

[FilipO28555]

This extension is based on papers:
Wu et al
PIC code Chicago
Cannoni
Cross section parametrization:
Bosh, Hale

There is now documentation.

Code flow:

                                                               ┌ IntraCollision.hpp (not yet tested/included - will be in the next PR)
simulation.hpp -> Fusion.x.cpp -> Collider.hpp -> WithPeer.hpp ┤
                                                               └ InterCollision.hpp

Struct InterCollision{} is a combination of algorithm from Collision Extension and Creation Kernel.

                   uses: (particles/fusion/detail) 
InterCollision.hpp --------------------------------> FusionFunctor.hpp -> FusionAlgorithm.hpp

FusionAlgorithm.hpp decides if fusion happens and chooses momenta of new particles.

The setup that works and tests this extension is at: Fusion_Setups/DT-nHe_hal8999

Some preliminary results:

To do in consecutive PRs:

• Add ci test
• Add intra-species fusion
• More cross-section interpolators

[pordyna]

Right now this reuses a lot of collision code simply by copying files and also somewhat modifying them. We should unify this, for example, by generalizing the collision kernels. I'm not sure if this should happen before this is merged. @psychocoderHPC what do you think?

[psychocoderHPC]

Right now this reuses a lot of collision code simply by copying files and also somewhat modifying them. We should unify this, for example, by generalizing the collision kernels. I'm not sure if this should happen before this is merged. @psychocoderHPC what do you think?

we should keep the code first seperate, after merging we have a baseline to validate if we break something. Than we should check what we can unify.
This way requires that we not stop after after it is merged.
Before next week I am most likely not possible to look into this pr.

[BrianMarre]

@FilipO28555 the CI is still complaining that some source code files are marked as executable, for example,

• include/picongpu/particles/fusion/detail/ListEntry.hpp
• include/picongpu/particles/fusion/WithPeer.hpp

only binaries and scripts should be marked as executable, please follow the directions in the CI-job output on how to fix this.

[PrometheusPi]

@FilipO28555 This is not yet a full review, but it would be great if you remove unused /comment-out code so that the number of added files gets less scary (right now it's 3.5k lines)

[PrometheusPi]

Please remove uncommented /unused code

[ikbuibui]

One option if you want to keep this code is to add it as an example and activate it. That way we get better CI testing as well.

[ikbuibui]

In general, we avoid merge commits
See here and here

[ikbuibui]

Also I'm not sure how/if possible it is anymore but it would have been useful to have a commit at the state where you simply copied code from the collision pipeline and made it compile, and then added the fusion changes on top with separate commits.