This enables default copy-assignment operator and copy constructor.
Also removed GraphBase::NeighborIterator::operator!=(), since it will be generated from the equality operator.
- Extract base graph logic from WeightedEdgeGraph into new Graph class
- Add LabelGraph to replace old Graph class used by LanParty solver
- Rename WeightedEdgeGraph to WeightGraph and change it to use new Graph class
- Update affected solvers to use new graph classes