BUG: Prevent unintended keys from appearing in data bar.

The defaultdict behavior was allowing both algo code and
TradingAlgorithm wrappers to add unintended keys.

Remove use of defaultdict in favor of a dictionary that explicitly
adds the values in tradesimulation, otherwise allow a KeyError
if the bar is indexed with a sid that doesn't exist.

Also, when iterating over the keys in the data bar, only return
those keys that have pricing data.