New pgraph, but to be replaced again
Create new version of pgraph_new

(Hopefully, the last)
Restructure directory tree and update file names

*.py files are now in the src/ directory; replace the old
main graph.py file with the new one, and temporarily keep
the old one as graph_old.py for reference
Complete basic graph functions

Update _queue to add an empty() function, and remove graph_old.py
Add graphviz integration