Refactor

- Make test graphs reusable
- Switch to using unittest
- Reimplement q-variables as a DSL element
- Take first steps towards building non-pygraphviz data structure, put helper functions inside this
- Clean up notebook to not pollute image directory

Author: cthoyt

docs/source/hierarchical.rst

@@ -0,0 +1,3 @@
+Hierarchical Graphs
+===================
+.. automodapi:: y0.hierarchical

docs/source/index.rst

@@ -18,6 +18,7 @@ y0 |release| Documentation
    tian_id
    transport
    lvdags
+   hierarchical
    struct
    cli