floating point imu code for bradwii

THIS CODE MAY CONTAIN BUGS

USE WITH CAUTION

This is a replacement of bradwii's imu code for hubsan and other quads It can be cut and pasted or simply replace the original file. There is no compass.

This was a small educational project to see if the truncation errors in the fixed point code of the original imu code cause instability. The type of rotations the code performs can be changed between small angle and normal type, with 3 levels of precision. Execution time is about 3ms compared to 1.7ms for the old code.

The accuracy of the code is improved compared to the original, and this translates in better flight characteristics.

If it is too big to compile:

• in Kiel uVision Options for target -> use microlib
• in Kiel uVision Options for target -> use crossmodule optimization
• use different approximations by setting SMALL_ANGLE_APPROX and associated options