Non-spatial meshes

[franzpoeschel]

Implements openPMD/openPMD-standard#193

TODO:

• Forward/backward compatibility? (of API + of created datasets): No compatibility
• Python docu
• Set openPMD version in written datasets as "2.0"
• Merge Add openPMD version 2.0 to supported API versions #1551 first

[franzpoeschel]

This PR should be split into two parts at some point:

1. Introduction of openPMD 2.0, together with some helper functions
2. Introduction of non-spatial meshes

The first part might then also be relevant for other PRs where breaking changes are introduced (e.g. #1493 use of abbreviated attributes by default in JSON backend)

EDIT: Done #1551

[ikbuibui]

I took a look over the PR and it seems okay to me. The interface fits for the binningPlugin use case. However the review isnt very thorough as I am unfamiliar with most of the code base and I don't know enough to follow some workflows.

[ikbuibui]

A small comment is that in the binningPlugin in PIConGPU i was using std::array<double, 7> to hold the dimensionality data. I see that the API requires vectors, even though it assumes the 7 SI Dimensions. Is it really required to be like this?

[franzpoeschel]

Thank you for reviewing, Tapish!

A small comment is that in the binningPlugin in PIConGPU i was using std::array<double, 7> to hold the dimensionality data. I see that the API requires vectors, even though it assumes the 7 SI Dimensions. Is it really required to be like this?

Can you point me to where exactly you mean? Within the context of this PR, std::vector is used for n-dimensional data, i.e. for a 3-dimensional mesh, you would specify three numbers in Mesh &setGridUnitSI(std::vector<double> const &gridUnitSI).

Beyond that, we have always had two representations for Units: std::array<double, 7> and std::map<UnitDimension, double>. I have taken the chance of this PR to make this a bit more systematic and provide setters of both kinds, as well as conversion functionality.

With this PR, std::vector may now additionally used to specify the unit per dimension: in Mesh &setGridUnitDimension(unit_representations::AsArrays const &gridUnitDimension);, the type AsArrays resolves to std::vector<std::array<double, 7>>. So, the use of std::array mostly stays.

[ikbuibui]

Yes, you are right. It was my misunderstanding. Looks good!