The tell method of the optimizer allows replacing the data now

This retains the last position of the sampler, and thus improves the initialization of the next fit.
This fixes issue #11.

bask/optimizer.py

@@ -75,8 +75,11 @@ def ask(self):
                 raise RuntimeError("Initialization is finished, but no model has been fit.")
             return self._next_x
 
-    def tell(self, x, y, fit=True, n_samples=5, gp_samples=100, gp_burnin=10, progress=False):
+    def tell(self, x, y, fit=True, replace=False, n_samples=5, gp_samples=100, gp_burnin=10, progress=False):
         # if y isn't a scalar it means we have been handed a batch of points
+        if replace:
+            self.Xi = []
+            self.yi = []
         if is_listlike(y) and is_2Dlistlike(x):
             self.Xi.extend(x)
             self.yi.extend(y)
@@ -91,7 +94,7 @@ def tell(self, x, y, fit=True, n_samples=5, gp_samples=100, gp_burnin=10, progre
         if fit and self._n_initial_points <= 0:
             with warnings.catch_warnings():
                 warnings.simplefilter("ignore")
-                if self.gp.pos_ is None:
+                if self.gp.pos_ is None or replace:
                     self.gp.fit(
                         self.space.transform(self.Xi),
                         self.yi,