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