Make laser output LASY compliant

[AlexanderSinn]

This PR makes the laser envelope output of HiPACE++ comply with the openPMD-standard laser envelope extension (https://github.com/openPMD/openPMD-standard/blob/upcoming-2.0.0/EXT_LaserEnvelope.md) used by LASY. Instead of the separate fields laser_real and laser_imag, a single complex field named laserEnvelope is output if it is specified in the input script with:

diagnostic.field_data = laserEnvelope # Ez ExmBy etc.

or

diagnostic.field_data = all

• 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 great, thanks for this PR!

[MaxThevenet]

WARNING: This PR changes the output format for the laser data, affecting all simulations with a laser pulse.