check that convert subset strata to dataframe if they are sf objects

[AdamCSmithCWS]

If the user feeds a filtered version of the strata map to the stratify function: e.g.,

strata_alt <- load_map("bbs_cws") %>%
  filter(bcr != 3)

s <- stratify(by = "bbs_cws", strata_custom = strata_alt, species = "Canada Goose")

everything works fine until the generate indices function where it throws the following error:

Error in 'as_tibble()":
! All columns in a tibble must be vectors.
X Column 'geom' is a 'sfc_MultiPolygon/sfc' object.

This error relates to the meta_strata object which gets included as an sf object instead of the data frame the functions expect.

Insert a check into stratify_() to make sure any geometry is dropped when creating the metastrata object.