Change GraphBase::NeighborIterator to use pointer to graph instead of reference

This enables default copy-assignment operator and copy constructor.
Also removed GraphBase::NeighborIterator::operator!=(), since it will be generated from the equality operator.

src/common/GraphBase.cpp

@@ -41,12 +41,12 @@ bool GraphBase::areAdjacent(const int vertex1, const int vertex2) const
 
 GraphBase::NeighborIterator GraphBase::begin(const int vertex) const
 {
-    return { *this, vertex };
+    return { this, vertex };
 }
 
 GraphBase::NeighborIterator GraphBase::end() const
 {
-    return { *this, -1 };
+    return { this, -1 };
 }
 
 int GraphBase::addVertexInternal()