Added some documentation for LossMapDBReader/LossMapDBWriter.
Added some documentation for LossMapDBReader/LossMapDBWriter.


Former-commit-id: 82cd130102ed5cbd3376b9de6d1e2aa66d198a65 [formerly 733477e78845c559d4e6880b41490252bf658d59]
Former-commit-id: 12edd489b3634d16e80a2180cc86cb15d698e789
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine