Add a new build() function to StaticDigraph (#68)

This function builds the digraph from an arc list that
contains pairs of integer indices from the range [0..n-1].
It is useful in the cases when you would like to build a
StaticDigraph from scratch, i.e. you do not want to build
another digraph that can be copied using the other build()
function.

 @nodes

 label

 0

 1

 2

 3

 4

 5

 6

 7

 @arcs

 		label capacity

 0 	1 	0  	  16

 0 	2 	1 	  12

 0 	3 	2 	  20

 1 	2 	3 	  10

 1 	4 	4 	  10

 1 	5 	5 	  13

 2 	3 	6 	  10

 2 	4 	7 	  8

 2 	6 	8 	  8

 5 	3 	9 	  20

 3 	6 	10 	  25

 4 	7 	11 	  15

 5 	7 	12 	  15

 6 	7 	13 	  18

 @attributes

 source 0

 target 7