Merge pull request #10012 from gschorcht/drivers_sht3x

drivers/Makefile.dep

drivers/Makefile.include

drivers/include/sht3x.h

drivers/sht3x/Makefile

drivers/sht3x/doc.txt

drivers/sht3x/include/sht3x_params.h

drivers/sht3x/sht3x.c

drivers/sht3x/sht3x_saul.c

sys/auto_init/auto_init.c

sys/auto_init/saul/auto_init_sht3x.c

tests/driver_sht3x/Makefile

tests/driver_sht3x/README.md

tests/driver_sht3x/main.c