def get_cell_indices(self):
return [(i,j) for i in range(self.num_rows)<EOL>for j in range(self.num_cols)]<EOL>
Return row and column indices for every cell in the grid.

This includes cells that aren't necessarily associated with any widget.  
For example, if a grid has a child in position (1,1) and none anywhere 
else, then it has 2 rows and columns and this method would return:
[(0,0), (0,1), (1,0), (1,1)]