Fix adaptive time step for non-electron beams

[AlexanderSinn]

The correct formula for the beam betatron angular frequency is omega_b = sqrt(q_beam * q_plasma * n_plasma / (2 * gamma_beam * m_beam * epsilon0))

Test with 8 GeV electron beam vs. 80 GeV anti proton beam:

• 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]

Thanks for this PR!