SourceModelLogicTree should treat source models independently

This is related to the new python logic tree implementation, see https://bugs.launchpad.net/openquake/+bug/857364

Source models listed on logictree's first branching level can technically have nothing in common. They can in particular have different source ids. On the parsing and validation step we need to make sure that filters in branchset reference only sources which exist in all source models that can lead to the branchset in question.

Currently the SourceModelLogicTree keeps source ids available for filtering (from all source models) in a plain set.