Make it possible to compute the GMFs directly from the ruptures stored in the database

[Michele Simionato]

This will allow

1) distribution by rupture
2) on-the-fly export of the GMFs, see https://bugs.launchpad.net/oq-engine/+bug/1321544

Actually it already possible to do so, but is inefficient to filter the ruptures: the idea instead is to save the filtered indices in the database, so that the filtered site collection can be rebuilt without any distance computation (notice that the distance computation will involve ALL the sites,
whereas the filtered sites will be likely much less than the total).
Adding a site_indices field to ProbabilisticRupture is the way to go.
A performance analysis in serious computations (like the full Japan) shows that the space required is negligible (both in memory and disk).

[Michele Simionato (michele-simionato)]

https://github.com/gem/oq-engine/pull/1441

[Michele Simionato (michele-simionato)]

https://github.com/gem/oq-engine/pull/1478