Changes compared to the original pull request in the drivers repository simplefoc/Arduino-FOC-drivers#12
1. Added a version of init which turns the motor one revolution to find the center values of the sensors.
2. Moved the calls to analogRead into a weakly bound function ReadLinearHalls so it can be overridden with custom ADC code on platforms with poor analogRead performance.
3. Commented out the pinMode calls in init, which makes it possible to pass in ADC channel numbers for custom ReadLinearHalls to use without having to remap them every update.
4. Changed to use the much faster _atan2 function that was added to foc_utils recently.
5. Added examples.

As requested by runger

Terrible bug in the fast sin/cos added last year, resulting in rough sine waves and a spike from +1 to -1 at pi/2

I thought I discarded this change after moving LinearHall back to drivers, but somehow it got included anyway