Replace sapply() by type safe vapply()

Avoid unexpected coercicion by sapply().

scripts/1_extract_metadata.R

@@ -168,8 +168,8 @@ process_sheet <- function(file, sheet) {
 
   ## From the first row of the file, extract...
   header <- strsplit(as.character(dat[1, ]), "\\$")
-  variables <- sapply(header, `[`, 1) # variable names
-  units <- sapply(header, `[`, 2) # units
+  variables <- vapply(header, `[`, 1, FUN.VALUE = character(1)) # variable names
+  units <- vapply(header, `[`, 2, FUN.VALUE = character(1)) # units
 
   ## Assign variable names as column names
   names(dat) <- variables