Merged from master
Introduced a namedtuple SourceRuptureSites
Merge pull request #1390 from gem/disagg-2

Optimize the disaggregation calculator
Merge branch 'master' of github.com:gem/oq-engine into light-site-collection
Merge pull request #1396 from gem/light-site-collection

Make use of the light site collection introduced in hazardlib
Merge branch 'master' into send-site-collection