Resolve "How should learners handle data that is outside of the domain"

Closes #101

See merge request !115

Resolve "LearnerND fails for BalancingLearner test"

Closes #105

See merge request !112

This test fails sometimes

Resolve "Learner1D's bound check algo in self.ask doesn't take self.data or self.pending_points"

Closes #95

See merge request !99

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

See merge request !107

Make the AverageLearner only return new points, and ignore
subsequent updates to existing points (as in the rest of
adaptive).

See merge request !104

move specific tests for a particular learner to separate files,
now that the number of tests is getting large.

See merge request !105

Now the filename tells us that the tests pertain to Learner1D.

move tests specifically for the 'Learner1D' to a seperate file and rename test_learner.py -> test_learners.py

Correctly set interpolated loss when points are added, taking care
of the cases when points are added outside of the known intervals.

Closes #99

See merge request !100