Implement expand operation (st_expand())

[jolars]

It would be great to have an implementation of st_expand() in line with the definition at https://postgis.net/docs/ST_Expand.html.

[edzer]

package stars has an unexported function stars:::bb_shrink that shrinks a bounding box with a factor; giving it a negative factor will expand it. Something like this could be used as a starting point.

[wiesehahn]

it's not implemented in sf, is it?

[edzer]

No.