[mini] Fix b0 calculation in collisions

[SeverinDiederichs]

This is a small PR to disentangle PR #954. This PR fixes the bug that was also fixed in #954 and in WarpX at ECP-WarpX/WarpX#3928

This PR needs to reset the benchmarks, so that then #954 should not touch the benchmark.

• 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