Namespace cleanup

[dshemetov]

Our namespace (as of the R6 refactor #431) looks something like

• clone
  • clone.epi_archive
  • clone.grouped_epi_archive
• group_by
  • group_by.epi_archive
  • group_by.epi_df
  • group_by.grouped_epi_archive
• print
  • print.epi_archive
  • print.epi_df
  • print.grouped_epi_archive
• slide
  • epi_archive
  • grouped_epi_archive
• epi_slide
• epix_as_of
• epix_merge
• epix_slide

It works, but it's a bit inconsistent. It would be nice to clean this up. Some ideas (from @brookslogan):

1. Continue to use epi_ vs. epix_. This has three sub-approaches to consider:

• Use generics epix_as_of, epix_slide, epix_fill... epix_truncate..., merge, clone
• Use above generics, but also have functions epix_merge and epix_clone that do basically the same thing as the generics (we may or may not want some sort of type checking, not sure)
• Use generics epix_as_of, epix_slide, epix_fill... epix_truncate..., epix_merge, epix_clone

2. Eliminate epi_ and epix_ prefixes.

• Use generics as_of, slide, fill...., truncate...., merge, clone.... except maybe as_of should be named better... and also slide. [rough ideas: version_time_slide, time_slide, snapshot_as_of]