Add Connection class, to allow for non-global use.

Preserve global APIs 'setup', 'execute_query', etc., using a global instance of that class.

'Vertex' and 'Element' subclasses can be assigned an instance via '_set_connection', or have one passed in the class suite as '_connection';  in either case the global instance will be ignored (and need not even be configured).

Making the classes independent of the connection would be even more flexible, but would involve moving all the class methods, as well as the instance construction, to the connection object.