Add Grid Ionization

[AlexanderSinn]

This PR computes the temperature and chi of electrons created by laser ionization on a grid in the limit of infinitely many particles per cell, without using any particles. Much faster and with no noise compared to using many particles per cell. Assumes that particles are stationary and no plasma wake forms.

Runtimes:

4 ppc: 2.4 s
100 ppc: 28.68 s
grid: 1.3 s

Laser:

Electron temperature:

Refractive index:

• 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]

ux^2 etc. should take into account the gamma/(psi+1) factor. Additionally, chi indeed does decrease outside the laser pulse. Both effects are tiny for a0=0.1 but noticeable for a0=1, if the formation of a wake is manually suppressed.