Graphical editor to create graphs (nodes and links only)
Editor fully done

To switch between modes, just type:
- 'n' to be in the NODE mode: when clicking you create a node
- 'l' to be in the LINK mode: you can create a link between node A
 to B by first clicking on A and then B. Clicking "on" take the
closest node
- 'o' to be in the ORIGIN mode: click on a node to transform it into
an origin. They will be displayed in red.
- 'd' to be in the DESTINATION mode: click on a node to transform it
into a destination. They will be displayed in blue.