Merge branch 'planetary' into 'master'

configure.ac

doc/RTD/source/Planetary/index.rst

doc/RTD/source/Planetary/initial_conditions.rst

src/equation_of_state/planetary/sesame.h

src/hydro/Minimal/hydro.h

src/hydro/Minimal/hydro_debug.h

src/hydro/Minimal/hydro_iact.h

src/hydro/Minimal/hydro_io.h

src/hydro/Minimal/hydro_part.h

src/hydro/Planetary/hydro.h

src/hydro/Planetary/hydro_iact.h

src/hydro/Planetary/hydro_io.h

src/hydro/Planetary/hydro_part.h

src/units.c

src/units.h