Move parallel edges algorithms to new Algorithm\Parallel

* [x] Move Set::hasEdgeParallel() to new Algorithm\Parallel::hasEdgeParallel()
* [x] Move Edge\Base::hasEdgeParallel() to Algorithm\Parallel::hasEdgeParallelEdge() and Edge\Base::getEdgesParallel() to Algorithm\Parallel::getEdgesParallel()
* [x] 100% code coverage
* [x] Fix getting parallel edges of undirected edges
* [x] Documentation / changelog

Refs #27.