Modify the implementation of ListDigraph::ArcIt (#311)

The new implementation is based on out-arc iteration (like
ListGraph::ArcIt) instead of in-arc iteration to make it
consistent with the documentation.

Much better implementation for node splitting (#311)
in ListDigraph. This solution is the same as the one that
is used in SmartDigraph. It is much faster and does not
invalidate any iterator like the former implementation.

Add a resize() function to HypercubeGraph (#311)
just like the similar functions in other static graph structures,
and extend the test files to check these functions.

Add reserve functions to ListGraph and SmartGraph (#311)
ListDigraph and SmartDigraph already have such functions.