reset cashed cholesky value if re-run computation oh hyperparameters

treegp/gp_interp.py

@@ -114,6 +114,7 @@ def _fit(self, kernel, X, y, y_err):
         :param y:  Values of the field.  (n_samples)
         :param y_err: Error of y. (n_samples)
         """
+        self._alpha = None
         if self.optimizer != "none":
             # Hyperparameters estimation using 2-point correlation
             # function information.