feat: added a Drift trait and a "default" ConstantDrift implementation

src/lib.rs

@@ -8,6 +8,7 @@ pub mod agent;
 #[cfg(feature = "approx")]
 mod approx;
 pub mod batch;
+pub mod drift;
 mod game;
 pub mod gaussian;
 mod history;
@@ -15,6 +16,7 @@ mod matrix;
 mod message;
 pub mod player;
 
+pub use drift::{ConstantDrift, Drift};
 pub use game::Game;
 pub use gaussian::Gaussian;
 pub use history::History;