Fixed the check constraint for the 'period' param and added some comments to explain the intention of the check logic.
Fixed the check constraint for the 'period' param and added some comments to explain the intention of the check logic.


Former-commit-id: 14a594cc88170d4f6ddbfc7b802b4078cd28e543
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine