Function axis_angle6 

pub fn axis_angle6<T: Float>(exp: &V6<T>) -> (V6<T>, T)

Convert a 6D vector of exponential coordinates into screw axis angle

Function arguments: exp: V6 A 6D vector of exponential coordinates for rigid-body motion S * theta

Output: (V6<Float, Float) a tuple with the first element the normalized “screw” axis, and the second tuple element called “theta” representing the distance traveled along/about S