Converts an sfg object to a data.frame
Examples
sfg <- sfg_point( obj = c(1,2) )
df <- sfg_to_df( sfg )
m <- cbind( matrix( 1:24, ncol = 2 ), c(rep(1, 6), rep(2, 6) ) )
sfg <- sfg_polygon( obj = m, x = 1, y = 2, linestring_id = 3 )
df <- sfg_to_df( sfg )