Add seed to write_beam.py to fix from_file test

[AlexanderSinn]

Previously it was possible for write_beam.py to generate a beam file that had a beam particle outside of the simulation box, causing the particle to be reset and the test to fail. This was a 5.7 sigma outlier, with a 10e-8 chance, but with 10e6 beam particles it occasionally happened.

• 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

[SeverinDiederichs]

Thanks for finding and fixing this!