Prefiltering sources in all calculators

See https://bugs.launchpad.net/oq-engine/+bug/1260658

This saves a lot of memory and is a pre-requisite for the "splitting area source->point sources" task, which requires keeping in memory all the sources. In other words, this pull request makes it possible to run on a laptop computations which an extremely large source model and a lot of realizations, provided one is interested on few locations.