feat(schema): derive JsonSchema for multi-network models

[Mearman]

pywr-cli export-schema writes the JSON Schema for ModelSchema, but the multi-network model types (MultiNetworkModelSchema and the types it contains) do not derive JsonSchema, so a multi-network file has no schema to validate against and cannot be part of a combined schema.

This adds the derive on those four types and nothing else, so a schema can be generated for them with schema_for! the same way as for ModelSchema. No behaviour changes: the derive only adds a trait implementation.

It is needed by #901, which uses it to export one schema for v1 and v2 model files. This change does not depend on that work.