Add individual Adafruit_Sensor instances for each of the LSM9DS0 board sensors to allow compatibility with AHRS algorithms.

Broke out read, get event, and get sensor function into individual functions per sensor.  Then added support for exposing each sensor (accelerometer, magnetometer, gyroscope, and temperature) as an Adafruit_Sensor class that is compatible with other code (like AHRS algorithms) which depends on unified sensor library inputs.