Feature node storage for Graph class (#347)

* Add nodeSet member for Graph class

* Use cached nodeSet in all algorithms

* Add test for graphs with isolated nodes

* Fix typo in overload of addEdge

* Formatting

* Fix type in graphTest

* Delete Edges linked to removed Node in removeNode

* Add more tests for addNode and removeNode methods

* Add test for getNode method

* Restore original use of nodeSet and cache only isolated nodes

* Add checks on size of isolatedNodesSet

* Add test of set data for isolated node

* Small fix