Add multigroup anisotropic scattering physics

[clemekay]

From the 05/07/2025 document "MC/DC Plans Through the End of CEMeNT" :

In our current MG test problems, we don't need anisotropic scattering. However, such capability would particularly be important if we were to expand to other particle transports that are highly anisotropic. Ganapol's recent analytical benchmark will be used for verification.