mctools/ncrystal: NCrystal release 2.6.1

Small bugfix release fixing rare numerical artifacts in some inelastic cross sections. This is not believed to have had a significant impact on any materials in the current data library, but the artifacts showed up in some of the materials being developed for the future v2.7.0 release.

Specifically, this release provides a workaround for numerical artifacts in inelastic cross sections arising from low-granularity beta-grids. In practice this affected materials which either used hard-coded low-granularity scattering kernels, or those with phonon VDOS curves that had large empty regions (for instance seen in hydrogen VDOS curves in materials with C-H bonds). The workaround for external low-granularity kernels consists of a simple "thickening" of the beta-grid, while the workaround for VDOS-based kernels consists in modifying the beta-grid generation code to ensure points are inserted into any large gaps in the single-phonon region.