Simplify SuccessionDiagram API

[jcrozum]

While building the docs, I noticed that the SuccessionDiagram API could be simplified in a few ways:

1. We have various node attributes that could be grouped into a separate object, e.g., sd.node_is_expanded should be sd.node.is_expanded.
2. Several of the from_xxx(s) methods could be grouped into one function, e.g., from_string(s, format=xxx).
3. The various expansion methods could be aggregated into one with an method argument.

[jcrozum]

One more thing to note about this class is that there is a circular import issue, and there are some troubles importing the class from the top level. We should rename SuccessionDiagram.py to succession_diagram.py and move the SuccessionDiagram import in the SCC expansion to within the function call (or even better, come up with a good way to avoid needing the import at all).

[jcrozum]

Note #106 addresses points 1 & 2 above, but not 3.