Merge pull request #1446 from gem/refactor-realizations

Removed the need for logictree.enumerate_paths
Fixed the bug in the risk event based calculator for multiple realizations
Merge pull request #1447 from gem/fix-gmf-duplication-2

Fix gmf duplication
Merge branch 'master' of github.com:gem/oq-engine into fix-event-based-risk
Merged from master