performance fix for _mean_based computation
performance fix for _mean_based computation


Former-commit-id: 2bd7208302d1bf61aadaea1a433e9fb6b9a1d160 [formerly 908fee2a1dc827ccc8deb618f584fe761769b5ca]
Former-commit-id: 943a3977dc3847e6844cdde6bf5a6bbcb979e0f8
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine