Be explicit about the attribute-geometry relationship of sf objects

[florisvdh]

This is needed to prevent warnings in geometry operations, assuming a constant attribute throughout each geometry. More information: https://r-spatial.org/book/05-Attributes.html, https://r-spatial.github.io/sf/reference/sf.html.

In many cases, adding %>% st_sf(agr = "identity") will be sufficient for the sf objects returned by n2khab.