particle checkpoints vs sampling
Particle "state" and "rhs" data is required for continuing DNS. It should be saved in double precision. Sampled fields come from single precision computations, it makes sense to save them in single precision. Also full particle trajectories only need to be saved in single precision as far as postprocessing is concerned, since the quantities we're interested in will not be affected by lack of precision.
What needs to happen is:
- modify HDF5 datatype used in _particles.h5 file for sampled fields.
- modify existing method into "checkpoint", and generate an adequate method for "sampling states" (i.e. write same data to different datasets).