distance sensors: Improve variable naming / description

msg/distance_sensor.msg

src/drivers/distance_sensor/leddar_one/leddar_one.cpp

src/drivers/distance_sensor/ll40ls/LidarLiteI2C.cpp

src/drivers/distance_sensor/mb12xx/mb12xx.cpp

src/drivers/distance_sensor/sf0x/sf0x.cpp

src/drivers/distance_sensor/sf1xx/sf1xx.cpp

src/drivers/distance_sensor/srf02/srf02.cpp

src/drivers/distance_sensor/teraranger/teraranger.cpp

src/drivers/distance_sensor/tfmini/tfmini.cpp

src/drivers/distance_sensor/vl53lxx/vl53lxx.cpp

src/drivers/px4flow/px4flow.cpp

src/modules/ekf2/ekf2_main.cpp

src/modules/simulator/simulator_mavlink.cpp

...ebop_rangefinder_wrapper/df_bebop_rangefinder_wrapper.cpp

...posix/drivers/df_isl29501_wrapper/df_isl29501_wrapper.cpp

...forms/posix/drivers/df_trone_wrapper/df_trone_wrapper.cpp