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

NOTEPAD.md

@@ -6,3 +6,11 @@ let mut history = History::new();
 let agent_a = history.new_agent();
 let agent_b = history.new_agent_with_prior(Prior::new(Gaussian::default(), BETA, GAMMA));
 ```
+
+```rust
+trait Team {
+    fn players(&self) -> impl Iterator<Item = P>;
+    fn weights(&self) -> impl Iterator<Item = f64>;
+    fn score(&self) -> u16;
+}
+```