Returns the given sudoku as a string of 81 givens.
str = sudoku_writegivens ( X )
a 9-by-9 matrix, with 0 for unknown entries
a string of 81 letters, with dots "." for unknowns. The entries come row by row.
Converts a sudoku in matrix form into a string, where "." represents unknowns.
http://www.sudocue.net/fileformats.php