gff reader now writes comments, invalid and ignored files; 1bp-long features allowed.
added os.remove to gff_reader_tests to clean up files
added tearDown method to gffreader tests
removed annotations-writing bit from gene_parts' 'write_to_tbl' function; we'll let mrna handle that.
removed 'hypothetical protein' bit from gene_part's 'write_tbl_entry' function; that is a parent's responsibility
mrna.to_tbl checks for presence of product; if not found, adds hypothetical protein