logictree: use existing function for getting path to nrml schema file
logictree: use existing function for getting path to nrml schema file

Former-commit-id: 4a0d3b74238626b9ad4bd851b4bb954707041a33 [formerly 4a0d3b74238626b9ad4bd851b4bb954707041a33 [formerly efc4adc1c3b5369886faffbd684e43c5501573aa]]
Former-commit-id: 5b4943ef32318724ebd303ad185194920dcbc52f
Former-commit-id: a43b91d636ccf05f9d06ee7cf588035d126b81e5
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine