Merge branch 'fix_minimal' into 'master'

Fixed the minimal SPH hydro version

Fix to #148. The code now runs without segfaulting when using the minimal SPH hydro code. Results are not great (by design) as the model is really poor (but simple).

Changes are:
 - A missing initialisation in `engine_init_particles()`.
 - A mistake in the calculation of the du/dt term.

See merge request !169