Improvements to i/o and parallel-i/o

This implements multiple changes to the i/o, especially the parallel version.

• Wrap particles back into the box before writing them (Fix to #374 (closed)).
• Use the threadpool to construct the internal buffers that are sent to HDF5.
• Make the buffer construction generic across all three i/o routines.
• Allow for more general functions to transform particles into i/o buffer quantities.
• Choose more optimised ROMIO algorithms for parallel-io and delay the writing of meta-data until the file is closed.

This last change allows for a writing speed of 6GB/s on the cosma-6 Lustre storage. (Solved #115 (closed))