Added a generic SourceConverter class for all sources, including the NonParametric ones

See https://bugs.launchpad.net/oq-engine/+bug/1364418. This requires two companion PRs on nrmllib https://github.com/gem/oq-nrmllib/pull/130 and https://github.com/gem/oq-nrmllib/pull/131 and one in the engine.
The tests are green: https://ci.openquake.org/job/zdevel_oq-commonlib/93/