Adds methods to retrieve forces added by the user.

[fabriceci]

My first rust commit, a very small addition to get the strengths/torques added by the user.

[fabriceci]

@sebcrozet The error comes from the file I did not modify, I get the same thing by running `cargo fmt --check".

There is a formatting problem in:

• examples2D/all_examples2.rs
• examples2D/rope_joints2.rs
• examples3d/rope_joints3.rs

[sebcrozet]

Thanks!