Fetch Openfisca YAML tests and add to Raputure DB

[Mischa]

• rake task (unique key = name)
  • new
  • updated
  • deleted
• basic fetch service
• Link scenarios and variables
  • map over each bottom layer of YAML inputs and outputs
  • check if it exists as a variable
  • collect all variable ids
  • add to join table variable-scenarios
• change rubocop Metrics/MethodLength to 20
• Add rspec tests for scenarios service

[Brenda Wallace]

Note: We will have duplicate names in open fisca -- but we shouldn't. There's an implied task here of finding incorrect and duplicate test names and fixing them in openfisca.