Adoption of openPMD in SPI simulation toolchain

[CFGrote]

openPMD should become the default metadata standard in most if not all modules. In the SPI toolchain, the PMI module writes to a custom format which is then picked up by the singfel diffractor.

• [ ] Develop an API (Calculator+Parameters) for XMDYN (gpu enabled full version)
• [ ] This API takes care of feeding wavefront data in legacy wpg format or openpmd format into the XMDYN run
• [ ] Consider adding the option to sample the focal spot (with fluence threshold)
• [ ] Convert XMDYN output into openpmd format (extension for electronic structure + MD data)
• [ ] Adopt SingFELPhotonDiffractor to read openpmd data from XMDYN API.