Enable ionization in normalized units

[SeverinDiederichs]

This PR follows #954 and only should be reviewed after #954 is merged.

This PR enables ionization in normalized units. Since the electric field in physical units is needed, the background plasma density must be provided in SI units via plasma.background_density_SI.

To do: proper assert that plasma.background_density_SI is provided in case of normalized units.

The difference between SI and normalized units is negligible:

• 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]

Looks good, thanks for this PR! See comment below.