Modifications dans area-based.1.data.preparation.Rmd

Adds lines : 14 et 15 erase all 211, 248, 441 : utiliser sf en lieu et place de sp qui ne supporte plus libproj6 area-based.1.data.preparation.Rmd

Suggestions integrated in push 94d51c3c, except for syntax lidR::projection(x) <- sf::st_crs(2154), which is replaced by lidR::projection(x) <- 2154

closed