NCrystal release 0.9.2

This release fixes a few important bugs in the NCrystal-Geant4 interface, which impacted the composition and neutronic cross-sections of NCrystal-enabled G4Materials. Only Geant4 users are affected by this change.

In addition, a few minor issues related purely to build & installation were addressed, concerning compiler warnings from GCC7 and relative RPATHs on OSX.