mctools/ncrystal: NCrystal release 3.4.0

This release modifies the installed location of CMake configuration files, to hopefully make it easier for downstream projects to locate an existing NCrystal installation, by hopefully making it unnecessary to set the NCrystal_DIR or CMAKE_PREFIX_PATH variables in many common scenarios (more details on related MCPL github issue https://github.com/mctools/mcpl/issues/58).

It also modifies the manner in which Geant4 bindings must be activated in downstream CMake code. Previously they were enabled via a dedicated find_package(G4NCrystal) call, but now they are instead provided as an optional COMPONENT when requesting the NCrystal package: find_package(NCrystal COMPONENTS GEANT4BINDINGS ) and if succesful the downstream binaries must be configured to depend on the G4NCrystal interface by adding the NCrystal::G4NCrystal target as a dependency, i.e. target_link_libraries( mybinary NCrystal::G4NCrystal ). The Geant4 bindings will not be enabled unless the GEANT4BINDINGS component is explicitly requested, and of course they must have been enabled in the first place with -DBUILD_G4HOOKS=ON when NCrystal was configured.