add 'save' and 'load' to the learners and periodic saving to the Runner

adaptive/learner/average_learner.py

adaptive/learner/balancing_learner.py

adaptive/learner/base_learner.py

adaptive/learner/data_saver.py

adaptive/learner/integrator_learner.py

adaptive/learner/learner1D.py

adaptive/learner/learner2D.py

adaptive/learner/learnerND.py

adaptive/runner.py

adaptive/tests/test_learners.py

adaptive/utils.py

docs/source/tutorial/tutorial.advanced-topics.rst

learner.ipynb