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)

Small improvements + add tests for StaticDigraph (#68)

Port StaticDigraph from SVN -r3524 (#68)

Small fix in the doc (#179)

Add citations to the min mean cycle classes (#179, #184)

Merge #179 (Port the min mean cycle algorithms)

Add tolerance() functions for MMC classes (#179)

Separate group for the min mean cycle classes (#179)

Simplify comparisons in min mean cycle classes (#179)
using extreme INF values instead of bool flags.

Add HartmannOrlin algorithm class (#179)
This algorithm is an improved version of Karp's original method,
it applies an efficient early termination scheme.
The interface is the same as Karp's and Howard's interface.

Add Karp algorithm class (#179)
based on the MinMeanCycle implementation in SVN -r3436.
The interface is reworked to be the same as Howard's interface.

Rename MinMeanCycle to Howard (#179)