learner tests fail

Currently several integrator learner tests fail.

To reproduce checkout current master (f268c8df as of now)

Running the test suite with the following random seeds will illustrate all of the failure modes:

pytest --randomly-dont-reorganize --randomly-seed=1531149508 adaptive/tests/test_cquad.py  # 2 failures in integrator learner
pytest --randomly-dont-reorganize --randomly-seed=1531150331 adaptive/tests/test_cquad.py  # 1 same failure as above, and 1 different

I will mark all these tests as xfailing for now, but IMO at least 1 of these failures does indicate a bug in the interval logic.