MAINT: Using dictionary literal syntax and combining lines

for readability
MAINT: Set initial values so to_dict can be called immediately

Previously, it would raise an exception.
MAINT: Remove unused members of TradingEnvironment.

The following members are no longer referenced elsewhere:
- `full_trading_day`
- `early_close_trading_day`
BUG: check that self.logger exists

`self.logger` is initialized as `None` and there is no guarantee that
users have set it, so check that it exists before trying to pass
messages to it.