Return x,y where x is a vector of sheet coordinates
        representing the x-center of each matrix cell, and y
        represents the corresponding y-center of the cell.

def sheetcoordinates_of_matrixidx(self):
        """
        Return x,y where x is a vector of sheet coordinates
        representing the x-center of each matrix cell, and y
        represents the corresponding y-center of the cell.
        """
        rows,cols = self.shape
        return self.matrixidx2sheet(np.arange(rows), np.arange(cols))