Support of the oq-lite command-line tool

See https://bugs.launchpad.net/oq-engine/+bug/1386665.
The tests are running here: https://ci.openquake.org/job/zdevel_oq-commonlib/161

There is also a companion on the engine: https://github.com/gem/oq-engine/pull/1570
is it different than having acc=AccumDict() in the function's signature?