Internal: Check input filetype.
checkQ(files = NULL, warn = FALSE)
A character or character vector of one or more input text files or a list of dataframes.
A logical indicating if a warning must be displayed for file those are not STRUCTURE, TESS or BASIC file.
A character or character vector indicating input type 'STRUCTURE', 'TESS', 'BASIC', 'CLUMPP', 'list', 'data.frame', 'UNIDENTIFIED' for all selected files.