Again energy step at restart (like #93)

Very similar to #93 (closed) (fixed by !254 (merged)).

cd doc/examples/forcing_anisotropic_3d
./run_simul_toro.py -N 40 -Rb 10 -nz 20 --t_end 0.2

Then in the simulation directory:

fluidsim-restart . --add-to-t_end 0.05
ipython --matplotlib -i -c "from fluidsim import load; sim = load(); sim.output.spatial_means.plot()"