sensors: Calculate and publish pre-flight IMU sensor consistency metric

If a single sensor is fitted, the calculation is not performed and zero values are published.
If dual IMU's are fitted, the vector length difference between the primary IMU and the second sensor is output for the angular rates and accelerations. The vector difference is low pass filtered before the length is taken.
If three IMU's are fitted, the vector length is calculated for both alternative sensors and and the maximum values output.
Fourth and subsequent IMU's are ignored.