Remove the table gmf_scenario and make the management of GMF consistent for all calculators

[Michele Simionato]

The existence of the gmf_agg tables means that now the gmf_scenario table can be removed, since we can store the GMF for the scenario calculator directly in gmf_agg. This has a substantial benefit for the developers: gmf_scenario is no more a special case and we can drop a lot of delicate code, in particular for the hazard_getters of the risk and their tests; moreover that managements of the outputs becomes consistent for all tables and tasks like https://bugs.launchpad.net/openquake/+bug/1163276 becomes easier. Having a set of consistent output containers make it possible to find general approach to face the complexity of the outputs, which has become a maintenance burden, with several open bugs.