Add Graph::write interface to write grap to file

Graph::write() writes graph to a file in DIMACS format.
There is an interface to create graph inside a program, one may want to
write the result to a file.
Fix bugs in Akiba et. al. algorithm implementation

Akiba class was designed to allow multiple executions but this was
broken due to bus. Now fixed and tested.
Add Labeling::get_label interface

Label is a list of hubs and a coresponding list of distances to hubs.
Labeling::get_label_hubs() and Labeling::get_label_distances() can be
used by user to get label for a vertex.