change the simplex_queue to a SortedKeyList

I suddenly realized the reason why the test_learner_performance_is_invariant_under_scaling was failing for the LearnerND sometimes.

We were saving the rounded loss and then when a subsimplex was created from an existing simplex, we divided the rounded loss instead of dividing the unrounded loss and then rounding it again.

Using a SortedKeyList will make sure this doesn't happen.

setup.py

@@ -48,7 +48,7 @@ setup(
     name='adaptive',
     description='Adaptive parallel sampling of mathematical functions',
     version=version,
-    url='https://gitlab.kwant-project.org/qt/adaptive',
+    url='https://adaptive.readthedocs.io/',
     author='Adaptive authors',
     license='BSD',
     classifiers=[