Separate grid for Laser

[AlexanderSinn]

In this PR, the grid of the laser is decoupled from the field's grid. To facilitate this, this PR...

• uses the laser geometry inside of laser functions
• adds interpolation for aabs.
• adds interpolation for chi.
• adds a way to get chi outside the fields grid.
• adds a check for laser functions if the current slice contains a laser.
• adds support for the laser MPI communication to skip slices outside the laser grid.
• changes the diagnostics to support getting the laser envelope on the laser grid and not the fields grid.

Test of getting chi outside the field:
Chi on the laser grid (changed diagnostic) with a slightly wrong box to transition between chi inside and outside the fields grid.

Chi on the laser grid with the correct box

Chi on the fields grid

• 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

[AlexanderSinn]

I changed how x, y, z is calculated here because the old way was wrong if the laser zeta axis was smaller compared to the full domain.

[MaxThevenet]

Thanks for this PR, looks great! See comments below. This is non-backward-compatible, right? @huixingjian could you give it a try for your simulations?

[AlexanderSinn]

This would be non-backwards compatible only because of the changes to diagnostic.base_geometry. The laser should work the same if the laser box is the same as level 0 (which is the default).