Examples, tests and notebook for the feature of oq-lite

This is the content of the course I gave yesterday. It requires the companion https://github.com/gem/oq-risklib/pull/81. The tests are running here: https://ci.openquake.org/job/zdevel_oq-commonlib/179
Missing values are okay and are converted into None. They are a wanted feature, to suggest to the user what are the available configurations she can tune. A missing key can be converted into None or can raise an exception, depending on the case at hand. The management of the keys is done in the oqvalidation.py module.