input/logictree:
input/logictree:

Refactored logic tree implementation a bit to support reading of logic
trees from the DB. Also added `read_logic_trees_from_db`.


Former-commit-id: ee4956908fec7e5dc63fe182c9b8cd8880ad8fee
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine