Add beam-plasma collisions

[SeverinDiederichs]

This PR extends the plasma collision module to do beam-plasma collisions

The implementation was tested against the theory in the following paper: Y. Yhao et al. Physics of Plasmas 27, 113105 (2020).

Using the same setup, Fig. 3 recreated with HiPACE++ looks like this:

Using more beam particles and more ions:

• 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

[MaxThevenet]

Looks good to me, thanks for this PR!

[SeverinDiederichs]

remove incorrect merging