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

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