Mesh refinement: enable longitudinal refinement

[SeverinDiederichs]

This PR enables longitudinal refinement for the refined level.
This 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 slices are calculated.

Additionally, in the InterpolateBoundaries, the correct longitudinal index is used in the coarse grid. This enables parallel runs.

This PR is tested in a parallel run in vacuum.

Additionally, I tested it with in a combined version with #561 and 564 in the blowout/ion motion case. However, the longitudinal refinement produces small step like ripples.

This was resolved by using a linear interpolation longitudinally. The fields Psi, Ez, Bz are also shifted to the Previous1 slice. Bx, By were already available. Then, a linear interpolation between the previous and the current slice is used. The ripples disappear:

• 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

[SeverinDiederichs]

outdated by #574