first preparation to enable multiple levels

[SeverinDiederichs]

This PR is a preparation for the upcoming implementation of mesh refinement.

It adds the ErrorEst function to tag the cells where the refinement is supported.
The lower and upper boundaries of the refined level can be read in via hipace.fine_tag_lo and hipace.fine_tag_hi.

The assert for lev = 0 is still in place and will be triggered if a simulation with max_level > 0 is started.

• 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.

[MaxThevenet]

Hm, just saw something, I don't think this should be merged actually.