update changelog for 2.0.0 release (engine-2.0)
update changelog for 2.0.0 release (engine-2.0)


Former-commit-id: 0db1f8b9138d11474c6f51feeb956deda29b8d55 [formerly 52593acc29de63da70058aa62804be2f1781841d]
Former-commit-id: 1566430b2a691c5edc379ac32b4fc901cde94098
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine