Also, add direct coverage of last_traded_dt in the `test_data_portal`
module.
Prepares for adding test coverage of `get_last_traded_dt` for `Future` assets.
Instead of having separate ExchangeCalendar and TradingSchedule objects, we
now just have TradingCalendar. The TradingCalendar keeps track of each
session (defined as a contiguous set of minutes between an open and a close).
It's also responsible for handling the grouping logic of any given minute
to its containing session, or the next/previous session if it's not a market
minute for the given calendar.
Instead of inferring it from the minute/daily writer, we now require the
first trading day to be passed explicitly, so the creator of the
DataPortal controls what is used as the first trading day.
Eddie Hebert