Sort outgoing arcs in the build() function of StaticDigraph

[Peter Kovacs]

In the build() function of StaticDigraph which takes another digraph as an argument, the outgoing arcs of each node are sorted with respect to their target nodes.

It clearly makes the build() function (and copying a graph to StaticDigraph) somewhat slower. Preliminary tests showed a difference of about 15 percent. On the other hand, it could make the usage of the constructed graph more efficient due to better caching. However, the trade-off is not clear.

I suggest to make this sorting optional (e.g. by adding a bool parameter to the build() function). However, the default option is of high importance here, since it will be used by DigraphCopy.

This ticket is a follow-up of #68.