handling for testing beyond the end of trading.environment's
history.
intraday risk and performance

both risk and performance now calculate performance since inception
(cumulative) and since the open. Both periods are updated intraday
and both are reported.

batch risk for periods starting after the
end of the treasury curve history now use
most recent curve.

took advice from PR by @ehebert, also switched to reuse the
iterative risk class.