Read a sudoku from a string of 81 givens.
X = sudoku_readgivens ( str )
a string of 81 letters, with dots "." for unknowns. The entries come row by row.
a 9-by-9 matrix, with 0 for unknown entries
Read a sudoku in a .sdk and return the matrix X associated, where 0 represent an unknown cell.
http://hodoku.sourceforge.net/en/docs_play.php#filters