Prevent perfectly centered plasma particle

[AlexanderSinn]

• 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]

Great, thanks! This fixes an issue identified by @AlexanderSinn: in a perfectly symmetric setup the macro-particle exactly on axis (which can happen with an odd number of cells) is on a very unstable position (for a symmetrical beam on axis), and would be kicked transversally in a random direction seeded by machine-precision noise, introducing randomness in a supposedly fully symmetrical and deterministic setup.