Save and export event loss data for specific assets

[Michele Simionato]

The idea is to have configuration parameters specific_assets and specific_assets_csv to specify the assets where the event loss should be stored.
Moreover we want to perform disaggregation only on the specified assets.

We should clarify what to do when the specific_assets file contains asset_refs not in the exposure (for instance due to a misprint).
Should we:

1) simply ignore the non-existing assets
2) print a warning
3) raise an error?

Moreover, if a rupture gives a null loss to an asset, should we save the zero or not? Saving a zero is wasteful, but at least it is unambiguous (an asset could not appear because of a bug, not because its loss is zero).

Should we add a demo demoing the new feature?

[Michele Simionato (michele-simionato)]

https://github.com/gem/oq-commonlib/pull/45
https://github.com/gem/oq-engine/pull/1564