As it makes the subsystems code easier to understand.
Declare the most used variables in main sketch file as forward declarations.
Make all control input zero by default (except controlMode).
Minor changes.
Add interpretControls function to convert pilot commands and mode into control targets and make control functions independent from the mode.
Add ratesExtra target for rates feed-forward; remove yawMode.
Rename controlRate to controlRates to reflect rates variable name.
Remove USER mode.
Make the order or basic methods consistent between Vector and Quaternion.
Remove `ZYX` from Euler method names as this is standard for robotics.
Rename angular rates to rotation vector, which is more correct.
Make rotation methods static, to keep the arguments order consistent.
Make `Quaternion::fromAxisAngle` accept Vector for axis.
Minor fixes.
