Move Vertex degree helpers to Algorithm\Degree

* [x] Move Vertex degree helpers to new Algorithm\Degree 
    * [x] Move Vertex::getDegree() to Algorithm\Degree:getDegreeVertex()
    * [x] Move Vertex::getDegreeIn() to Algorithm\Degree:getDegreeInVertex()
    * [x] Move Vertex::getDegreeOut() to Algorithm\Degree:getDegreeOutVertex()
    * [x] Move Vertex::isSink() to Algorithm\Degree:isVertexSink()
    * [x] Move Vertex::isSource() to Algorithm\Degree:isVertexSource()
    * [x] Move Vertex::isIsolated() to Algorithm\Degree::isVertexIsolated()
* [x] Update all references
* [x] 100% code coverage
* [x] Documentation / changelog

Supersedes #29.
Refs #27.