Uses kernel smoothing to smooth polygon sets
poly_smooth(polygon_set, by = "subid", smoothness = 3, min_points = 5)a data frame containing polygons, separated by `subid`
a character containing the name of the column grouping polygons to be smoothed. Default is subid
a numeric indicating the smoothing parameter, passed to `smooth_ksmooth`
a numeric indicating the minimum number of points to smooth. If a polygon has less than this number of points, it is not smoothed.
a polygon set with smoothed coordinates