always use PhysConstSI in parser

[SeverinDiederichs]

Previously, the constants such as clight etc in the parser were chosen according to the unit system. However, that prevents the most prevalent use case, where quantities in SI units (such as beam width or emittance) can be transformed into normalized units using the parser and the constants. This did not work because then the constants would also be normalized.

I don't see any use case, where normalized constants are needed (they can simply be omitted).

• 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!