Reindex sections based on indices

[N-Wouda]

Suppose my VRPLIB file has the following section:

NODE_SECTION
1  40  50
3  20  20
2  20  10

VRPLIB parses this as [[40, 50], [20, 20], [20, 10]], which ignores that the third and second node are provided in a different order in the data file. Thus, the parsed data are subtly wrong.

Additionally, the following

NODE_SECTION
1  40  50
2  20  20
4  20  10

is parsed in the same manner, despite node 3 never having been specified.

If we reindex the data based on the node indices, we get the correct order for free, and additionally, the second example will raise about 4 being out of range for the actual data. That means such an issue no longer fails silently.