mkexp: changed 'get_file' function to return unknown variable references unchanged, added 'get_dir' function, returning the path for a given 'files' section

mkexp: changed 'list' filter replacement to map the empty string to an empty list. Added option to keep empty string as singleton list if needed

mkexp: reading of EXP_DESCRIPTION from initial comment now also discards leading and trailing lines that only contain white space and comment characters