small refactoring in logictrees: filename is now objects' property
small refactoring in logictrees: filename is now objects' property


Former-commit-id: 01ad1a4b60d98d1748b0ae92abbb64d03beea9c2 [formerly f937ccf63f22be8680cad159d1542958ddcbaa1b]
Former-commit-id: fa20bb69d1f907ec5b2cd5fbd757547fc5f97411
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine