crop extent in `make_africa_mask` when `countries` is not `NULL`

[geryan]

e.g. the below is necessary becase the first call uses the extent for all Africa which is dumb

This function needs redoing wholesale I reckon. Come on mate lift your game.

make_africa_mask(
      type = "raster",
      res = "high",
      countries = "COD"
    ) |>
      crop(
        y = make_africa_mask(
          type = "vector",
          countries = "COD"
        )
      )

[geryan]

See #46

[geryan]

resolved by #72