use serial openPMD

[SeverinDiederichs]

We compile openPMD with the or without MPI according to whether HiPACE++ itself is compiled with or without MPI. However, IO is always serial, so this is not needed.

Ran into a compilation issue with ADIOS2 with MPI on LUMI, there I noticed this.

• 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]

Let's make this an option.

The reason for that is: let's assume you want to use HiPACE++ in a software environment with other BLAST codes that use openPMD, too. Some might depend on a parallel HDF5. But HDF5 unfortunately cannot be installed in two binary variants at the same time in environments like conda.

Similarly, an HPC system might only provide us with a parallel HDF5 (or only a serial HDF5) module.

In such situations, we want to control the parallelism of the I/O layer, even if we only use it serially.

Simple solution that I prefer:
we add an option(...).

Fancy solution:
we do a find_package(HDF5 ...) and decide based on this (but this is more involved and does not apply to other backends like ADIOS2)

[SeverinDiederichs]

I agree, let's do the simple solution. Will update the PR when I have time :)
Thanks!

[MaxThevenet]

Thanks for this PR!