Provide a way to avoid name prefix on df output from `f` in `epix_slide`

[brookslogan]

library(epiprocess)
#> 
#> Attaching package: 'epiprocess'
#> The following object is masked from 'package:stats':
#> 
#>     filter
library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union
library(data.table)
#> 
#> Attaching package: 'data.table'
#> The following objects are masked from 'package:dplyr':
#> 
#>     between, first, last
data.table(geo_value = c(1,1,1,1,2), time_value = 1, version = 1, value=1:5) %>%
  as_epi_archive() %>%
  epix_slide(function(x,y) tibble(geo_value = x$geo_value, value_sum = sum(x$value)), group_by=c(), n = 365000L)
#> Warning: Getting data as of some "clobberable" version that might be hotfixed,
#> synced, or otherwise replaced later with different data using the same version
#> tag.  Thus, the snapshot that we produce here might not be reproducible later.
#> See `?epi_archive` for more info and `?epix_as_of` on how to muffle.
#> # A tibble: 2 × 3
#>   time_value slide_value_geo_value slide_value_value_sum
#>        <dbl>                 <dbl>                 <int>
#> 1          1                     1                     9
#> 2          1                     2                     9

^{Created on 2022-10-06 by the reprex package (v2.0.1)}

This example is taken from another issue, but the takeaway here is the awkward default names, and passing in "", NA, or NULL to new_col_name has no effect.

[brookslogan]

This functionality already exists: names_sep=NULL. Though we might want to make things more dplyr-like by not prefixing to begin with, eliminating as_list_col and let users just wrap their output in a list if they want the corresponding output column to be a list, and eliminating names_sep.