Get the number of mean neighbors of spatial weights

`mean_neighbors(gda_w)`

- gda_w
A Weight object

The number of mean neighbors of spatial weights

```
if (FALSE) {
library(sf)
guerry_path <- system.file("extdata", "Guerry.shp", package = "rgeoda")
guerry <- st_read(guerry_path)
queen_w <- queen_weights(guerry)
mean_neighbors(queen_w)
}
```