Provide B4C and Gd2O3 for completeness.

[tkittel]

Although B4C and Gd2O3 are already available via the solid::B4C/... syntax, they lack the automatic density calculation found in https://mctools.github.io/simplebuild-dgcode/ (at least for B4C). Perhaps a few simple .ncmat files is all that is needed?

It might also be use for such strongly absorbing materials (and for gasmix: materials), to be able to simply use NCrystal as a composition calculator - i.e. let NCrystal create the material for the frontend MC app, but not add any "ncrystal::Scatter" property to it. A flag like prefer_native_scattering=1, which would have a meaning in OpenMC/Geant4 but be ignored in standalone NCrystal, McStas, etc.

[tkittel]

As concerns prefer_native_scattering, it might be worth considering if some frontends (openmc, geant4, ...) should perhaps default to that in case a material is anyway modelled directly as a free-gas model (i.e. for gasmix:: materials).

[tkittel]

Related for Gd2O3 to #147