Remove dependencies to maptools and rgdal

 Example:

```r
library(sf) # Adds as.owin.sf and as.ppp.sf methods
library(terra)
library(spatstat)

ma <- st_read("MA.shp")
p <- st_read("Walmarts.shp")
pop  <- rast("pop_sqmile.tif")

ma_owin <- as.owin(ma)
plot(ma_owin)
p_ppp <- as.ppp(p)
marks(p_ppp) <- NULL
plot(p_ppp)
Window(p_ppp) <- ma_owin
plot(p_ppp)
m <- as.data.frame(pop,xy=TRUE)
im <- as.im(m)
image(im)
```