fix for potential CRS problems on older systems

[andysouth]

Older systems can have problems with the way CoordinateReferenceSystem is saved

We could maybe provide an auto run function that resets the CRS of afrilearndata objects on the users system ?

This worked to fix afrimapr/afrilearnr#2 (comment)

sf::st_crs(africapitals) <- 4326
sf::st_crs(afrihighway) <- 4326
sf::st_crs(africountries) <- 4326
sf::st_crs(africontinent) <- 4326

see :
r-spatial/sf#1419 (comment)