Return loss matrix

Companion to https://github.com/gem/oq-risklib/pull/74. Now the event based risk calculator uses less memory than before. The tests are green: https://ci.openquake.org/job/zdevel_oq-engine/585
Please ignore the changes to the experimental calculator event_based_fr
mop, notice when I write "Please ignore the changes to the experimental calculator event_based_fr"

This calculator is not yet mature. If it is noisy, I can remove it from the codebase and commit it when it is ready.