setup CRAN repository

requirements.R

+# CRAN repository
+local({
+    r <- getOption("repos")
+    r["CRAN"] <- "http://cran.r-project.org"
+    options(repos=r)
+})
+
 # manage R packages with pacman R package
 if(!requireNamespace("pacman", quietly = TRUE)) install.packages("pacman")