[WIP] openPMD: Handle fields with Guards

[ax3l]

For the corner-case of enabling guards in m_F (through hipace.3d_on_host = 0), we need to copy the validbox data out before we can present contiguous memory for I/O.

To do:

• GPU test is writing zero data with this patch (even for hipace.3d_on_host = 1)

Normal check list:

• 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

[ax3l]

something here is off in GPU runs

[SeverinDiederichs]

This PR works fine, however it is based on the broken PR #270, therefore I will close it here and re-opened it as #272. #270 was reverted in #273.