Adjoint solution with random ray

[jtramm]

Adjoint solutions are generally very easy to generate with random ray, basically just requiring the transposition of the scattering matrix but otherwise performing transport as normal. Adjoints have a number of uses, but the one we are most interested in for the short term is for use with the FW-CADIS algorithm for weight window generation. This is not expected to be a hard feature to add, but should be implemented with the idea that both the forward and adjoint solutions may need to be generated at once so they are both available when generating weight windows.