Remove unused D_to_e function.
Try reading file in 1 pass.
No need to seek to the beginning of the dataset...
Delete count_nodes, count_elements, and beginning_of_dataset functions.
Remove order_of_datasets vector.
Remove need_D_to_e flag.
Rename node_{in,out} and element_{in,out} functions.
Remove unused _n_nodes member.
Remove unused _n_elements member.