Simplify Query 15 renaming logic

[ChrisJar]

Currently query 15 relies on a column renaming to both map the column names to new names and to flatten from a multi-index to a single level index. i.e.

regression_groups.columns = regression_groups.columns.map(
        {
            ("x", "count"): "count_x",
            ("x", "sum"): "sum_x",
            ("xx", "sum"): "sum_xx",
            ("xy", "sum"): "sum_xy",
            ("y", "count"): "count_y",
            ("y", "sum"): "sum_y",
        }
    )

It'd be great to simplify the logic leading up to this to avoid relying on complex index naming changes.