Save only non zero losses in the table event_loss_asset

[Michele Simionato]

This provides a tremendous speedup, so big that it is worth the risk of not knowing if an asset is missing because of a bug or because it is zero. Otherwise computations would be impossibly slow.

[Michele Simionato (michele-simionato)]

https://github.com/gem/oq-engine/pull/1675

[Michele Simionato (michele-simionato)]

https://github.com/gem/oq-risklib/pull/167

[Michele Simionato (michele-simionato)]

Also, the exported event_loss_asset CSV file should have the same order in the test than when exported:

https://github.com/gem/oq-risklib/blob/save-non-zero-losses/openquake/qa_tests_data/event_based_risk/case_2/expected/event_loss_asset.csv