[LSM9DS1] added updateTimestamp for magneto / various comments

src/shared/sensors/LSM9DS1/LSM9DS1_AxelGyro.cpp

src/shared/sensors/LSM9DS1/LSM9DS1_AxelGyro.h

src/shared/sensors/LSM9DS1/LSM9DS1_Data.h

src/shared/sensors/LSM9DS1/LSM9DS1_Magneto.cpp

src/shared/sensors/LSM9DS1/LSM9DS1_Magneto.h

src/tests/drivers/test-lsm9ds1-fifo.cpp

src/tests/drivers/test-lsm9ds1.cpp