Add layer structure for graph

[Premmie]

In order to display the graph properly an ordering based on layers has to be made. This will improve the readability of the graph
13.3 Explains the concepts
https://cs.brown.edu/~rt/gdhandbook/chapters/hierarchical.pdf
http://chianti.ucsd.edu/svn/coreplugins/tags/cyto-2_3/AutomaticLayout/src/csplugins/layout/algorithms/hierarchicalLayout/Graph.java

[Jipje]

Horizontal layer structure has been added, as well as some data that can be used in the vertical layering of the nodes. See VisualAlgo or Canvas branches