Include external control interface for heating rod

[FelixStege]

Closes #132

[ebc-aixlib-bot]

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• HTML syntax of your models, primarily in your documentation.
• Adherence to the naming convention in all changed files.
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• If your contribution changes existing reference results.

If HTML errors occur, I will fix the issues using a separate pull request.
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Tips to fix possible naming violations:

• Stick to the naming guidelines, e.g. Namespace Requirements
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  replaceable package Medium = Modelica.Media.Interfaces.PartialMedium "Medium model";
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  Tip: To create the initial script, you can use Dymolas script generator, explained here: https://www.claytex.com/tech-blog/how-to-use-a-plot-script/)

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[HvanderStok]

Looks good!