Graph: update clear function

* Call clear function of InternalGraph.
* Clear the instance count.
Graph: add getNbNodes and getNbConnections
pyTest: add testDeleteNode
pyTest: add testClearGraph
Graph: refactoring - rename

* Rename _nodes to _nodesMap.
* Rename functions getNodes() by getNodesMap().
Graph: add getNodes

* GetNodes() functions return a vector of Nodes.
* Note: could easily iterate on result in python binding.
InternalGraph: add getters to connected and unconnected vertices

* Add getConnectedVertices function.
* Add getUnconnectedVertices function.
* Add include to boost/graph/connected_components.
InternalGraph: update removeUnconnectedVertices

Use getUnconnectedVertices function.
Graph: add getters to connected and unconnected nodes

* Add getConnectedNodes function.
* Add getUnconnectedNodes function.
Graph: add deleteUnconnectedNodes
pyTest: add testDeleteUnconnectedNodes
InternalGraph: refactoring - suppress unused typename
Merge pull request #347 from cchampet/dev_Graph

Graph: add functions to delete nodes