Mesh refinement: fix boxarray

[SeverinDiederichs]

This PR fixes a few things:

the BoxArray on level 1 is now correct. It was already correct transversely, but not longitudinally. Here, we construct it from the correct transverse part and the longitudinal part from level 0. DefineSliceGDB is now called per level and adapted correctly.

This PR does not fix that the plasma etc, which use some of the variables touched here are initialized per level. They are still just initialized on level 0, this will be fixed in an upcoming PR.

Majority of this PR was authored by @MaxThevenet.

• 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