Change time to use i64 instead of u64

src/gaussian.rs

@@ -42,7 +42,7 @@ impl Gaussian {
         }
     }
 
-    pub(crate) fn forget(&self, gamma: f64, t: u64) -> Self {
+    pub(crate) fn forget(&self, gamma: f64, t: i64) -> Self {
         Self {
             mu: self.mu,
             sigma: (self.sigma.powi(2) + t as f64 * gamma.powi(2)).sqrt(),