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
I made init_tags, since it is an implementation defailt, so the docstring can be omitted :-)