Consider adding helper functions to get default `ref_time_values`, etc.

[brookslogan]

This would make it easier to write tests and for users to understand. Using these as default values for optional parameters rather than using the missing approach would also make it quicker to find than wading through code. We could choose not to export these functions to avoid bloating the namespace but still allowing users to use ::: to access them.

[brookslogan]

Interacts with #99.

[brookslogan]

This is easy to do for epix_slide ref_time_values without duplicating code, but not so simple for epi_slide ref_time_values (it arranges the df then takes the unique time values, but still wants the arranged df around; looks like there would be a performance or maintenance cost here). I'll just implement, unexported, the epix_slide ref_time_values default for now.