Make `Vec3<T>` a special, augmented version of `Vec<T,3>`

The following discussion from !3359 (merged) should be addressed:

Vec3<T> should become a Vec<T,3> with additional functionality. Same thing for Lorentz 4-vectors (which should encode both the ordering convention: x,y,z,t / px,py,pz,E and the metric convention eg diag(+1,+1,+1,-1) (?)

changed the description

mentioned in merge request Analysis!842 (merged)

After !3907 (merged) is merged, the only remaining place where Vec3 is used is the VeloUT reconstruction algorithm. Once Vec3 are replaced in it, we'll be able to remove it.

The reason this is not removed in VeloUT is that there the old-style accessors are used, and it should be converted to the SOACollection?

yes

Rec#456 (closed)