changing Society.source to foreign key on Source model
working around a typecast issue
adding sources file for loader
adding source field to EnvironmentalValue
adding source field to EnvironmentalValue
adding source to more models
updating tests
adding sources to environmental
making an empty isocode return None too
adding some rudimentary caching to go nice on the database
adding source to binford data
updating source info in EA societies
adding transaction management to speed up load.py