Merge branch 'serial_io' into 'master'

Allow the code to do i/o in serial while MPI is activated

Quite a big change. Main goal is to solve issue #29 and hence offer a workaround #22.

The old non-MPI code is now in single_io.[ch] and is called when WITH_MPI is not defined.

The old MPI code with parallel-HDF5 is called when WITH_MPI and HAVE_PARALLEL_HDF5 are both defined.

The new MPI code is in serial_io.[ch] and is celled when WITH_MPI is defined but HAVE_PARALLEL_HDF5 is not.

In this serial mode, the code forces the different MPI ranks to write to the same file one after the other, appending their particles at the end of the pre-existing set of particles.

This merge request also corrects issue #27 and a bug in the attributes of the arrays in HDF5 that got the wrong units.

SWIFT now runs with Platform-MPI !!

See merge request !14


Former-commit-id: 8c6aaed9c5fbc30e205a7246838b7013a7be4bdc