Changed the EventBasedGMFXMLWriter to use the node library
Merge branch 'master' of github.com:gem/oq-nrmllib
Merge branch 'master' into export-gmf-save-memory
Merge branch 'master' of github.com:gem/oq-nrmllib
Merge branch 'master' into export-gmf-save-memory
Fixed the ScenarioWriter and the tests
Merge pull request #114 from gem/export-gmf-save-memory

Exporting the GMF in XML should not require keeping all the tree in memory