Merge branch 'logger_add_periodicity' into 'master'

logger/Makefile.am

logger/chemistry/GEAR/logger_chemistry.h

logger/chemistry/none/logger_chemistry.h

logger/gravity/MultiSoftening/logger_gravity.h

logger/hydro/Gadget2/logger_hydro.h

logger/hydro/SPHENIX/logger_hydro.h

logger/logger_index.c

logger/logger_interpolation.h

logger/logger_parameters.c

logger/logger_parameters.h

logger/logger_particle.c

logger/logger_particle.h

logger/logger_python_wrapper.c

logger/logger_reader.c

logger/logger_reader.h

logger/star_formation/GEAR/logger_star_formation.h

logger/star_formation/none/logger_star_formation.h

logger/stars/Basic/logger_stars.h

logger/stars/GEAR/logger_stars.h

src/gravity/MultiSoftening/gravity_logger.h