Check laser input file contains a normalized vector potential

[AlexanderSinn]

By default, LASY outputs the electric field instead of the normalized vector potential, which previously HiPACE++ would have misinterpreted as a normalized vector potential, causing it to be off by a factor of 10^12.

Additionally, the lasers.lambda0 was not documented under the form_file init type, even though it has to be set for all laser init types.

• 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