Enable longitudinal refinement

[SeverinDiederichs]

This PR enables longitudinal refinement for the refined level. It is a clean re-opening of #565.
It is based on #572 and should only be reviewed after #572 is merged

Longitudinal is done in the following way:

The beam is binned per level. In the loop over the slices of the coarse grid, a sub-loop over the refined grid is done. For a refinement ratio of 4, 4 sub-slices are calculated.

• 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]

Thanks for this PR!
Oops some conflicts appeared, do you think you could fix them?

[MaxThevenet]

Thanks for this PR! Just have 1 style comment, which applies to a few places in the code.

[MaxThevenet]

I am not sure this is safe across compilers and configurations. Instead of

f( // comment
    a, b);

Could you do one of these?

f( /* comment */
    a, b);
// comment
f(
    a, b);