Added a method PMF.sample_pairs
Added a method PMF.sample_pairs

gem/oq-engine,gem/oq-engine,gem/oq-engine,vup1120/oq-hazardlib,vup1120/oq-hazardlib,vup1120/oq-hazardlib,gem/oq-hazardlib,gem/oq-hazardlib,rcgee/oq-hazardlib,rcgee/oq-hazardlib,gem/oq-engine,gem/oq-engine,gem/oq-hazardlib