implemented mapping between desired thrust and applied pwm in

src/drivers/px4io/px4io.cpp

src/modules/px4iofirmware/mixer.cpp

src/modules/px4iofirmware/protocol.h

src/modules/px4iofirmware/px4io.h

src/modules/px4iofirmware/registers.c

src/modules/sensors/sensor_params.c

src/modules/systemlib/mixer/mixer.h

src/modules/systemlib/mixer/mixer_group.cpp

src/modules/systemlib/mixer/mixer_multirotor.cpp