Enhancements for network class

[SimonNick]

I tried to improve the network class a little bit. @IngoScholtes , we should have a look at it when you are back.

• Nodes do not have the attributes for weights and degree any more. One can use the respective functions degree(), in_degree(), out_degree(), weight(), in_weight(), out_weight().
• Rewrote add_node(), remove_node(), add_edge() and added the method remove_edge() for a better readability.
• Use @property for the properties directed, vcount, ecount, weighted, total_edge_weight. This makes also sure that properties like directed cannot easily be changed.
• Added methods in_edges() and out_edges() which obtain all in- respectively out-edges of a given node.
• Added methods to_directed() and to_undirected() which convert network into directed / undirected network.
• Added methods read_nodes() and write_nodes() which read / write node data to a file.
  Rewrote methods read_edges() and write_edges() such that they also read / store edge attributes.

[IngoScholtes]

Thanks, I will have a look and we can discuss next week