Added a "node_factory" validation facility to the node library

This is here to support the work on nonparametric sources (https://bugs.launchpad.net/oq-engine/+bug/1364418).
By writing a suitable "node_factory" in commonlib it is possible to convert all sources from NRML to hazardlib objects in few lines of code. That means that in the future nrmllib could disappear. 
The tests are green: https://ci.openquake.org/job/zdevel_oq-nrmllib/43