List of files from directories/files list or browser

This functions goal generate a cell array of file names, needed by the calling functions. The input should be a cell array with directories including the files or files name. The function also supports input of a single directory name string, or a single file name string. In the late two cases, each file is tested for existence. Absolute file path is used thus replacing the relative path. Alternatively, the user can choose the files or directories including files using the OS explorer- by enabling the 'flagGUI' input.