like that, sounds very transparent
so the structure including the time step would be like:
`fields/<t>/fieldE/<data|ghost>/x,y,z`
`particles/<t>/spezies0/<data|ghost>/x,y,z`
or
`<t>/fields/fieldE/<data|ghost>/x,y,z`
`<t>/particles/spezies0/<data|ghost>/x,y,z`
?

(I would still encode the time step, even if we are going to open a new file per time step)