Use consistent code style in R-tutorial notebooks

Working on #377, I noticed that the code in the R tutorials (at least) is not consistently formatted.

Well-formatted code increases readability, which is critical for new users. In contrast, inconsistently-formatted code adds the (avoidable) difficulty of reading the code to the (inevitable) challenge of learning the core concepts of the modeling framework.

Choose a style guide and ensure all code in the tutorial notebooks conform to it.

Locate the R tutorial notebooks referenced in the issue and review their current formatting alongside the project context from #377. Choose and document a consistent style guide, then apply it across the R tutorial code; done means all code in those notebooks follows the selected style consistently.

Written by the indexing model from the issue text.