Improve input validation in read_PSL2R()

[mcol]

Function read_PSL2R() accepts either a single .psl file as input or a path to a directory where .psl files are searched. Note that we enter the second case also if a file with an extension other than .psl is given:

file <- list.files(file, pattern = ".psl$", full.names = TRUE, ignore.case = TRUE)
if (!all(file.exists(file)))
  stop(...)

If a directory path is given and if no .psl files are found, list.files() returns character(0), and in turn !all(file.exists(file)) is FALSE and the function doesn't stop!

The same failure occurs if the directory doesn't exist in the first place:

> read_PSL2R(file = system.file("extdata/non-existent-directory", package = "Luminescence"))
[read_PSL2R(): The following files were found and imported: 
 .. 
Error in file(con, "r") : invalid 'description' argument