review of alternate sampling I/O

In b3e45e20 I made some changes to the particles_output_sampling_hdf5 class (and related code), such that when I do need to sample particle data, the particle file stays open. My understanding is that HDF5 is much more efficient if a file is kept open, since the buffer is only flushed when full. @bbramas , can you please review my changes and confirm that I didn't do anything stupid (in particular with the temporary data arrays, and the particle positions). In principle look at the latest commit in the "feature/efficient-particle-sampler" branch, since I may tweak it soon.