Introduce 'tell_pending' which replaces 'tell(x, None)'

and I also tackle

• lower_bound <= learner.ask()[0] <= upper_bound
• if we tell(x, y) with x outside the bounds, the point should be added to data, but not any of the other internal datastructures. This will mean that the point will be "invisible" to the learning algorithm, and only useful for plotting and data extraction

for both the Learner1D and Learner2D in that MR. I included this in here to avoid merge conflicts later.

adaptive/learner/base_learner.py

@@ -44,6 +44,12 @@ class BaseLearner(metaclass=abc.ABCMeta):
         for x, y in zip(xs, ys):
             self.tell(x, y)
 
+    @abc.abstractmethod
+    def tell_pending(self, x):
+        """Tell the learner that 'x' has been requested such
+        that it's not suggested again."""
+        pass
+
     @abc.abstractmethod
     def remove_unfinished(self):
         """Remove uncomputed data from the learner."""
@@ -62,14 +68,14 @@ class BaseLearner(metaclass=abc.ABCMeta):
         """
 
     @abc.abstractmethod
-    def ask(self, n, add_data=True):
+    def ask(self, n, tell_pending=True):
         """Choose the next 'n' points to evaluate.
 
         Parameters
         ----------
         n : int
             The number of points to choose.
-        add_data : bool, default: True
+        tell_pending : bool, default: True
             If True, add the chosen points to this
             learner's 'data' with 'None' for the 'y'
             values. Set this to False if you do not