radiation plugin does not produce openPMD output anymore

[PrometheusPi]

The current dev version of PIConGPU does not produce openPMD radiation output anymore.
Most likely this is caused by #4387. A simulation that used the 2022-11-22 dev branched produced the openPMD files.

[PrometheusPi]

Discussion with @franzpoeschel:
In his case, the files were created. I will test on a different cluster.

[PrometheusPi]

@franzpoeschel found out that this is only triggered with checkpoints are also written.

[PrometheusPi]

If checkpoints are active - radiation data is stored in the checkpoint directory.

[franzpoeschel]

I was not aware that checkpointing uses the same instance of the plugin as normal output. Since the recent PR keeps one instance of the openPMD::Series open, whatever call to the plugin comes first – normal output or checkpointing – gets the shot at deciding where output will go.
Should be easy to fix by making the plugin keep two instances open – one for normal output, another one for checkpoints.

[PrometheusPi]

fixed with #4401