Mesh Refinement with Explicit Solver

[AlexanderSinn]

A few things had to be adjusted for MR to work with the explicit solver:

• One MG solver per level.
• Set the boundary condition for Bx and By on level 1. This is similar compared to PC but the MG solvers (hpmg and amrex) expect the boundary at the cell face instead of center, so a few values had to be adjusted.
• Properly shift slices for level 1.
• Shift the domain box in hpmg so that lo is at 0,0,0. Contrary to level 0, level 1 doesn’t already have this property.
• Assert that MR is not used with SALAME.

This fixes the last open issue in #667

• 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]

Looks good to me, thanks! See minor comments & questions below.
@WeiqunZhang this PR changes a few things in hpmg, could you have a quick look? Thanks!

[WeiqunZhang]

LGTM