add classical radiation reactions for beams

[SeverinDiederichs]

This PR adds the energy loss due to classical radiation reactions of particle beams to the code.
The implementation is based on the following paper: M. Tamburini et al., NJP 12, 123005. In the paper, the implementation is discussed for a Boris pusher. HiPACE++ uses a velocity Verlet pusher for the beam particles.
This document translates the approach from the paper to a velocity Verlet pusher.

To benchmark the implementation, it was tested against the theory from the following paper: P. Michel et al., PRE 74, 026501, where the energy loss due to betatron radiation in a blowout is discussed. Using the same test case as in CI (just with more output to give a meaningful plot and more beam particles) gave the following result (similar to Fig. 1 in the paper):

• Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
• Tested (describe the tests in the PR description)
• Runs on GPU (basic: the code compiles and run well with the new module)
• Contains an automated test (checksum and/or comparison with theory)
• Documented: all elements (classes and their members, functions, namespaces, etc.) are documented
• Constified (All that can be const is const)
• Code is clean (no unwanted comments, )
• Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
• Proper label and GitHub project, if applicable

[AlexanderSinn]

Can you calculate RRcoeff outside of the kernel and also make it work in normalized units using plasmas.background_density_SI ?

[SeverinDiederichs]

I am going to add normalized units in a separate PR, since this will require shifting the plasmas.background_density_SI to hipace.background_density_SI and I found this to be cleaner in a separate PR.

Edit: Added it already now

[AlexanderSinn]

Can you include HiPACE++ with radiation reactions turned off in the plot in the PR description?

[SeverinDiederichs]

Can you include HiPACE++ with radiation reactions turned off in the plot in the PR description?

I added it, since it is a matched beam, nothing happens without RR

[SeverinDiederichs]

In the end, normalized units are not that much extra effort, so I included them in this PR. They work well:

[MaxThevenet]

Great, thanks for this PR! See minor comments below.