function angleDelta(a, b) {
    return modHalf(b - a, twoPI);
}
angular delta between angle 'a' and 'b'
solution taken from: https://stackoverflow.com/a/2007279

@param {number} a : first angle in *radians*
@param {number} b : second angle in *radians*
@return {number} angular delta in *radians*