GraphNode must be refactored

[wouterjsmit]

Possible points for refactoring are the following.

• Remove the Collection API for in-edges, out-edges and genomes.
  These are not needed, since we can simply provide the Collection<> interface via the get() methods. This does increase the coupling, because Collection<> is returned rather than some implementation.
• Create a single Position and/or GuiData class and store it in GraphNode, rather than loose fields.
  GraphNode in general is getting very bloated with fields that are required for the GUI. Another option to fix this would be to create a class that wraps the GraphNode and contains these fields (this would be some GuiNode). The second option will most likely be preferable, because that keeps the data model decoupled from the GUI (rather than putting GUI in the model, we put model in the GUI).
• The GraphNode test suite, which is currently contained in SequenceNodeTest must be generalized to be run for every type of GraphNode, including bubbles.