make getting highest_period more general
make getting highest_period more general


Former-commit-id: 514cbad2e605ae30c20c825e5ab291831b15ac67 [formerly 830e693780f804f603889a6671d3bb57072fc741]
Former-commit-id: 2be029e6cd51ba8cb2ce62b88bef0dfa8b81e68f
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine