Merge branch 'sphenix_sph' into 'master'

configure.ac

doc/RTD/source/HydroSchemes/anarchy_sph.rst

doc/RTD/source/HydroSchemes/index.rst

doc/RTD/source/HydroSchemes/sphenix_sph.rst

src/Makefile.am

src/debug.c

src/hydro.h

src/hydro/SPHENIX/hydro.h

src/hydro/SPHENIX/hydro_debug.h

src/hydro/SPHENIX/hydro_iact.h

src/hydro/SPHENIX/hydro_io.h

src/hydro/SPHENIX/hydro_parameters.h

src/hydro/SPHENIX/hydro_part.h

src/hydro_io.h

src/hydro_parameters.h

src/part.h

tests/test125cells.c

tests/test27cells.c

tests/testActivePair.c

tests/testInteractions.c