Merge remote-tracking branch 'origin/master' into threadpool_task_plots2

.gitignore

src/engine.c

src/hydro_properties.c

theory/Multipoles/cells.pdf

theory/Multipoles/fmm_summary.tex

theory/Multipoles/gravity_derivatives.tex

theory/Multipoles/mesh_summary.tex

theory/Multipoles/plot_mesh.py

theory/Multipoles/plot_potential.py

theory/Multipoles/potential_derivatives.tex

theory/Multipoles/potential_softening.tex

theory/Multipoles/run.sh