Remove length unit scaling from NORMAL/UNIFORM direction cosines

[GuySten]

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Fixes #344

Description

The direction cosines for the NORMAL/UNIFORM distributions were scaled by length_unit.
This PR removes the length_unit scaling from the direction cosines in the UNIFORM/NORMAL distribution cases.

Tests

cargo test passes on my machine. (Ubuntu 24.04.1 LTS (WSL2 on Windows 11, x86_64), rustc 1.98.1).
After this fix the MWE from #344 works correctly.

❯ RustBCA 0D mwe_angstrom.toml
❯ awk -F, 'NR==1{x=$4;y=$5;z=$6} NR==3{dx=$4-x;dy=$5-y;dz=$6-z;n=sqrt(dx*dx+dy*dy+dz*dz);printf "%.4f %.4f %.4f\n",dx/n,dy/n,dz/n; exit}' angstrom_trajectories.output
0.5809 0.8140 0.0000

Generative AI Use

Claude helped find the bug and the fix, and generated the MWE and the awk command.

[drobnyjt]

@GuySten Thank you very much!