Consider enforcing a consistent output row ordering in `epi_slide`, `epix_slide`

[brookslogan]

Currently, epi_slide probably arranges by group vars & time_value, while epix_slide likely arranges by (ref_)time_value & group vars.

[brookslogan]

Not sure we want these to match between epi_slide and epix_slide. In the slide improvements pass there is the lingering question of whether we should actually reorder the columns to better "match" the different row ordering there.

[brookslogan]

Not sure if the column ordering was migrated to another issue from this checklist. Leaving open for now.