Allow for creating a LookupDef without manually initializing a ModelListing

[chrispcampbell]

One thing I didn't like about the recent changes to allow overriding lookup data (issue #472, PR #490) is that it requires the developer to create a ModelListing instance at runtime and manually find a VarSpec for the purposes of identifying the variable to be overridden.

Both ModelListing and VarSpec were previously implementation (or hidden API) details that were meant for advanced use cases (like in testing tools for accessing implementation variables), and I think it was a mistake to make developers have to understand these details.

It would be more convenient for developers if they can simply provide a variable name or ID to identify the variable.

Current:

  // Read the JSON model listing
  const listingJson = await readFile(joinPath('sde-prep', 'build', 'processed.json'), 'utf8')
  const listing = new ModelListing(listingJson)

  // Find the `VarSpec` that corresponds to a certain variable
  const lookupVarSpec = listing.varSpecs.get('_a_data[_a1]')

  // Create a `LookupDef` to override data for a variable
  const lookupDef = createLookupDef(lookupVarSpec, [...])

Proposed:

  // Create a `LookupDef` to override data for a variable (can now simply provide the variable name)
  const lookupDef = createLookupDef({ varName: 'A data[A1]' }, [...])

  // Or you can provide the variable ID
  const lookupDef = createLookupDef({ varId: '_a_data[_a1]' }, [...])

For this to work, it will be necessary to bundle a minimal model listing with the generated model code. This feature (overriding lookup data) isn't likely to be needed for many models and bundling the listing would increase the size of the generated code, so I think I will make it an opt-in thing for now.

Once this change is implemented, ModelListing and VarSpec can go back to being implementation details and can be hidden from the public API for the time being.