Implemented losses per event per asset

See https://bugs.launchpad.net/oq-engine/+bug/1383291 and the companion PR https://github.com/gem/oq-commonlib/pull/45. The tests are running here: https://ci.openquake.org/job/zdevel_oq-engine/766
Just a suggestion to remove some redundancy:
```
par_list = ['quantile_loss_curves', 'poes_disagg', 'poes_disagg', 'sites_disagg', 'sites_disagg', 'specific_assets', 'conditional_loss_poes']
for par in par_list:
    if par not in params:
        params[par] = []
```