Add picmi_schema

[mafshari64]

Add picmi_schema for picmi input files.

• do not merge before picongpu 0.8.0 branch is created (psychocoderHPC)

[psychocoderHPC]

Does PICMI somehow use this schema or is this an independent file.
How do we ensure that PICME is following the schema.

[mafshari64]

this schema in principle should check if the input file created based on the PICMI standard follows its role, eg units are correct.

[psychocoderHPC]

But how is the PICMI file checked against the schema, I do not see an integration into the PICMI python code.

[BrianMarre]

Some general remarks:

• the name of this schema is misleading since this schema has nothing to do with PICMI,

rather it describes the structure of the PyPIConGPU json description of an picongpu setup assumed by the meta data exporter, therefore it belongs to the meta data exporter and should use it's own project name to avoid confusion with the PyPIConGPU schemas.

• this schema is mostly a repeat of the PyPIConGPU schemas, which already specify the structure of the PyPIConGPU json description.
• this schema has nothing to do with PIConGPU and belongs to the meta data exporter project, since it describes assumptions/requirements of the meta data exporter not PIConGPU, PyPIConGPU or the PIConGPU PICMI implementation.