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.

Extend the interface of StaticDigraph (#68)
with index(), arc() and node() functions similarly to
other static graph structures.

Add documentation for StaticDigraph (#68)