This commit removes the ability to reference a shared TradingEnvironment through the zipline.finance.trading module. In place, the classes that require a TradingEnvironment, or its child AssetFinder, contain their own references to those objects.
This commit also adds serialization utilities that allow for the pickling/unpickling of objects without unintentionally their TradingEnvironments or AssetFinders.
on the number of per-tick update that occur since they were duplicated
per each PerformancePeriod. Also opens up the path to cythonizing the
entire object
jfkirk