i/o: python reader for checkpoint files

[germasch]

While checkpoints are mostly for checkpointing, obviously it's nice to be able to analyze the data inside, too, and having an interface that again makes the field data available as xarray, and the particle data in pandas would be useful. To dream aloud, there is a pandas - dask thing, too, that can allow to deal with datasets that don't fit into memory, though for now just being able to specify a subset of the domain for which particles should be ready would already go a long way (This pretty much exists already, but should be integrated into the psc python package.)