self loops are actually allowed in multigraphs
set_record method, and a bit of cleanup to prepare for loaders
move the mappings to graph.py because I expect them to make reference to the graph classes
fix imports
Keep "global" dictionaries where I can get at them
Cache will be mostly my own objects, rather than further dicts as before. Also, set branch and rev with __init__
moving stuff from the toplevel ORM class to those classes I'll be keeping instances of in the cache
schema changes
use the new global type-dicts
graph creation and access
Firm up the class hierarchy here