Compute the cells visible from cell a.
V = sudoku_visiblefrom ( a )
a 1x2 matrix, where a(1) is a row index and a(2) is a column index
a 9x9 boolean matrix. V(i,j) is true if the cell (i,j) is visible from cell a.
Returns the matrix of cells which are visible from both the cell a. The cell p is visible from the cell a if it is in the same row, column or box.