MAINT: Store values for market open and close in environment.

Instead of creating the market open and close mid-simulation,
calculate upfront the values for market open and close in a
DataFrame, so that they values can be looked up by date, as
viewed as series while investigating data issues.

One downside of this implementation is that the entire history
has open and close values calculated, even though the simulation
may only be a subset of the trade data on record.
Should consider moving the `times` property and other methods
that care about the start and end date of a simulation to
SimulationParameters or another like object.