Add arg function signature

[WardBrian]

This exposes arg, a function which was already documented but not in the signatures file. It closes #1000

Submission Checklist

• Run unit tests
• Documentation
  • OR, no user-facing changes were made

Release notes

Expose complex function arg

Copyright and Licensing

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license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[rok-cesnovar]

This is meant for 2.28.1 right?

[WardBrian]

If possible yeah. It was just an oversight that it got left out - it's in the doc of 2.28 already

[rok-cesnovar]

One simple question. I would love to see a model that uses the arg function as well. Can be a separate model or just a call to arg() with data and parameters as input in an existing model.