Improve Vector3::azimuthalAngle() (also used by FourMomentum and FourVector) to use exact rather than fuzzy is-zero check, to only check the perpendicular components, and to note that IEEE floating point implementations of atan2 should already be 'safe' unless we decide that this function should throw or return NaN in case of null or along-z vectors. Thanks to Louis Moureaux from CMS for the report and diagnosis.