Hi there! I really enjoy OpenFisca, but I recently encountered an issue. ### Here is what I did: Called `ParameterNode.get_descendants` in a tree with a parameter scale. ### Here is what I expected to happen: Expected an iterator including rates and thresholds from `ParameterScale` parameters. ### Here is what actually happened: The iterator didn't include the scale parameters. ### Here is data (or links to it) that can help you reproduce this issue: I've written a MWE [here on Google Colab](https://colab.research.google.com/drive/1umefvgCkl4Crvk0vScX-O8gkZvL5OUHZ#scrollTo=pywhkBUlKDlO). ## Context I identify more as a: - Developer _(I create tools that use the existing OpenFisca code)_. Thanks! Happy to tackle this with a PR if desired. (cc @benjello @sandcha)