Add unit test for Discrete-RBPF

[THargreaves]

Write two unit tests to verify the version of the RBPF using the forward algorithm as the inner algorithm (on both the CPU and GPU).

Note, this can be validated by making a DummyDiscreteModel similar to the DummyLinearGaussianModel where e.g. 9 states are partitioned into 3 outer, 3 inner states with observations only depending on the inner states. A fully CPU forward algorithm can be compared to a RBPF on CPU/GPU.