Generate SDMX data structures for the model comparison

[khaeru]

Deferred from #62.

Use the metadata about models to generate:

• Entries in the iTEM AgencyScheme, i.e. one Organization object for each iTEM-linked organization.
• A representation for the MODEL dimension enumerated by these.
• A codelist for the SCENARIO dimension containing the Scenario names.
  • Annotations including the reference/policy designator and the iTEM MIP to which the scenario is associated.