Standard deviation of losses in risk event based calculator

[matley]

Probabilistic Risk Event Based should also compute and save the standard deviation of losses (over the events) for each asset and for the whole exposure.

Proposed implementation:

1) add a field for storing standard deviation of loss ratios in the table riskr.loss_curve_data
2) add a field for storing standard deviation of losses in the table riskr.aggregate_loss_curve_data
3) compute the standard deviation by using numpy in the engine
4) Add a new output_type `event_loss_curve` for curves with standard deviation of losses

[matley (matley)]

related: https://bugs.launchpad.net/oq-nrmllib/+bug/1216870

[matley (matley)]

https://github.com/gem/oq-engine/pull/1264

[matley (matley)]

Also related: https://bugs.launchpad.net/oq-risklib/+bug/1216995