Provide a function for
-coloring a graph with integer marks.
The provided function is more efficient that the one provided by functor
val coloring :
Coloring.G.t -> int -> unit
coloring g k
colors the nodes of graph
using k colors,
assigning the marks integer values between 1 and k.
when there is no possible coloring.
The graph marks may be partially set before starting; the meaning of
initial values is as follows:
- 0: a node to be colored
- any value between 1 and k: a color already assigned
- any value greater than k: a node to be ignored