Remove clight factor from u

[AlexanderSinn]

This PR removes the factor of clight from the momentum arrays of beam and plasma particles. Whereas before the proper velocity was stored in u, now it is the normalized momentum = gamma * beta. This is much cleaner because it matches what is output to the user, saving many conversions between the two formats. In exchange, the particle pusher now normalizes the electric field with 1/c.

This seems to give a slight performance improvement:
PR

TinyProfiler total time across processes [min...avg...max]: 10.45 ... 10.45 ... 10.45
TinyProfiler total time across processes [min...avg...max]: 10.43 ... 10.43 ... 10.43
TinyProfiler total time across processes [min...avg...max]: 10.4 ... 10.4 ... 10.4
TinyProfiler total time across processes [min...avg...max]: 10.42 ... 10.42 ... 10.42

dev

TinyProfiler total time across processes [min...avg...max]: 10.56 ... 10.56 ... 10.56
TinyProfiler total time across processes [min...avg...max]: 10.51 ... 10.51 ... 10.51
TinyProfiler total time across processes [min...avg...max]: 10.51 ... 10.51 ... 10.51
TinyProfiler total time across processes [min...avg...max]: 10.51 ... 10.51 ... 10.51

I made sure that radiation reactions still work even in normalized units with the constants canceled out:

• 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]

Nice cleaning, thanks for this PR!