Add chunkwise to PS and CS API

[Helveg]

Users will probably often have to read out large quantities of data, too large to fit into memory at the same time (eg postprocessing, streaming to simulation backends, ...). To facilitate this, we could add a .chunkwise() method:

for ps in master_ps.chunkwise():
  # Do what you need with the set
  block_of_positions = ps.load_positions()