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.
MAINT: Factor out downside risk calculation.

Move the downside risk calculation into the main risk module;
so that the same calculation can eventually be used by both
the period and cumulative calculations, to prevent implementation
drift.
Add symbol to __all__

Currently symbol is not in __all__. So added it.
STY: PEP8 fixes.
TST: Disable talib default params test.

The data zipline_transform.window is always evaluating to empty,
thus the actual checks are not being used because of the
`if not data` done before running the `asserts`.

This behavior should be fixed, and we should either remove the
`not data` check, or bubble up that the check is being hit too
many times; but in the meantime, disabling this test which takes
a non-trivial amount of time to run.

When run as an algorithm, outside of unit tests the talib wrapper
does work, so the cause of the window always being empty may be to
due to the machinery of the unit test.
MAINT: Use current_data instead of last_sale_price

It seems more clear to get price values from
`self.trading_client.current_data[sid].price` than
from `self.portfolio.positions[sid].last_sale_price`.

The two values are the same, so this is just a readability change,
but it is also the same behavior as in `self.order_value()` and it's
good to have them all be the same.
BUG: Fix misalignment of downside series calc when using exact dates.

An oddity that was exposed while working on making the return series
passed to the risk module more exact, the series comparison between
the returns and mean returns was unbalanced, because the mean returns
were not masked down to the downside data points; however, in most,
if not all cases this was papered over by the call to `.valid()`
MAINT: Change variable name in test transforms comprehension.

Using `n` conflicts with using `n` in an interactive debugger, like
pdb. Use `name` instead.
MAINT: Remove unused setup from trading calendar test.

The values in the setup are unused in the test suite, so remove.

Also remove imports.