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

src/gaussian.rs

@@ -40,10 +40,10 @@ impl Gaussian {
         }
     }
 
-    pub(crate) fn forget(&self, gamma: f64, t: i64) -> Self {
+    pub(crate) fn forget(&self, variance_delta: f64) -> Self {
         Self {
             mu: self.mu,
-            sigma: (self.sigma.powi(2) + t as f64 * gamma.powi(2)).sqrt(),
+            sigma: (self.sigma.powi(2) + variance_delta).sqrt(),
         }
     }
 }