schema changed. added stddev_loss
added writers support for computing stddev
fixed migration. adapted calculators to use LossCurveCollection. Added regression test for stddev of losses
added event_loss_curve
fix qa test. loss_curve -> event_loss_curve
fix typo in the schema
updated tests
updated risk export test for dealing with event_loss_curve output type
removed unused import
Merge pull request #1264 from matley/std-losses

Compute standard deviation of losses