Bug fix in connectivity_summary; extend the API of create_png()

*connectivity_summary() does not use SummaryGraph correctly:
 Recently we changed the internal representation of SummaryGraph.ops, but
 connectivity_summary() and connectivity_summary_verbose() were not updated.
 Fixed that.

*Extend the API of create_png():
 Add to the signature of create_png() and create_pydot_graph() rankdir and
 External styles.  These are explained in the docstrings.

*Added documentation to the PNG drawing functions

*Added tests to catch trivial connectivity_summary() bugs