Merged nrmllib inside commonlib

See https://bugs.launchpad.net/oq-engine/+bug/1379827
The tests are green: https://ci.openquake.org/job/zdevel_oq-commonlib/141/
Notice the companion PR on the engine: https://github.com/gem/oq-engine/pull/1549
? I do not understand. The signature is `source`, the docstring seems correct to me.