Revise/nx graph

[uvchik]

Revision of the graph module

• I could not see the advantage of using a solph model instead of the energy system as the solph model gets its information from the energy system. That's why I removed the option to pass a model instead of an energy system.
• I added a label to the nodes which helps to show the description in external tools
• I added the weight to edges if an energy system is passed
• I added the option to write the graph to disc as an .graphml file.

Things we may want to discuss

• Do you like the integrated "write" option or do want to leave it to the user?
• At them moment no "weight" attribute is written if the weight is None. We also could add an empty attribute. I think both ways have advantages.

[uvchik]

I had to fix my PR, because I forgot that we are not allowed to change the API for v0.2.1.
The optimisation_model attribute will now raise a FutureWarning, but will still work.
Here are the Travis results: https://travis-ci.org/oemof/oemof/builds/330299021

[jnnr]

Looks good and gives nice results. Thanks! I like the labeling and weights. The integrated write function can be useful sometimes. If not needed, one just does not pass a filename, so that's perfectly ok.

The docstring could be revised. Under "Parameters", "filename" appears twice. Should I fix it?

[uvchik]

The docstring could be revised. Under "Parameters", "filename" appears twice. Should I fix it?

Are you sure you have the latest commit?

[jnnr]

Somehow I didn't. Strange. But now its ok.