Set a default height for the graph, expose width and height to the user

[martin-schlipf]

The height of the Graph in Jupyter notebook is rather small so that the plots do not look pleasing on typical desktop computers. We should set a higher default height (around 500px) to make them look better.
We can also make width and height attributes of the Graph object, so that users can set them to their liking.

Think about exposing all Graph attributes via kwargs to the user, e.g., you could allow

calc.dos.plot(xlabel="...")

and this would have the same effect as

graph = calc.dos.plot()
graph.xlabel = "..."