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/data_saver.py

@@ -32,10 +32,13 @@ class DataSaver:
         return getattr(self.learner, attr)
 
     def tell(self, x, result):
-        y = self.arg_picker(result) if result is not None else None
+        y = self.arg_picker(result)
         self.extra_data[x] = result
         self.learner.tell(x, y)
 
+    def tell_pending(self, x):
+        self.learner.tell_pending(x)
+
 
 def _ds(learner_type, arg_picker, *args, **kwargs):
     args = args[2:]  # functools.partial passes the first 2 arguments in 'args'!