Add support to simpler dataset format with indexing

[hftsoi]

Alternative input dataset format, say a single array ((idx=1, x_1, y_1, y_up_1, y_down_1, bin_width_1), (idx=2, x_2, y_2, y_up_2, y_down_2, bin_width_2), ...(idx=n, x_n, y_n, y_up_n, y_down_n, bin_width_n)), where all are put into the same place for each data point.
This could be useful for simplicity when handling 2d or high dim data, say (idx, x0, x1, y, y_up, y_down) etc.