Graph.Pack: parse_dot_file numbers from 1, not from 0

[eponier]

Contrary to Rand which starts from 0. This implies that doing in sequence dot_output then parse_dot_file produces a graph with nodes numbered differently. The culprit is

ocamlgraph/src/pack.ml

Line 149 in 726e8d6

incr new_node;

.

[backtracking]

You're right, that's inconsistent. It would be easy to fix (replacing "let new_node = ref 0" with 1 instead) but I'm a bit afraid it would break existing codes. Still hesitating...