mctools/ncrystal: NCrystal release 3.4.1

Very small release which exposes the C++-level utilities for completely flattening a material composition to the Python interface, as a new info.getFlattenedComposition() method. This is intended to better support materials with non-natural compositions in the upcoming OpenMC bindings. Internally, the new method is using the same machinery as what is already in place to provide the same functionality for the Geant4 bindings