Create aloft coverage function

[peterdesmet]

Before downloading vpts data from the aloft repository with download_vpts_aloft() (see #553), it might be useful for people to know what years/dates are covered for what radars. The repository does include a simple coverage.csv that lists all the files per directory:

baltrad/daily/bejab/2023,17
baltrad/hdf5/bejab/2018/05/18,47
baltrad/hdf5/bejab/2018/05/22,167
baltrad/hdf5/bejab/2018/05/23,229
baltrad/hdf5/bejab/2018/05/24,227
baltrad/hdf5/bejab/2018/05/25,215
baltrad/hdf5/bejab/2018/05/26,192
...

It might be useful to create a function that can return this information as a dataframe or visualize it immediately, e.g.:

show_aloft_coverage(
  # cf. download_vpts_aloft
  date_min = NULL, # limits columns
  date_max = NULL, # limits columns
  radars = c(), # limits rows
  format = "csv",
  source = "baltrad",

  # new params
  by = "year", "month", "day", # granularity of returned data
  show = "files", "boolean" # whether to indicate number of files or boolean
)

Alternatively, this functionality could be included in download_vpts(), but I'm not sure how.

Suggested output: