Random Ray Alternative Volume Estimators

[jtramm]

Currently the random ray implementation uses the "simulation averaged" volume estimator. This is useful as it avoids a ratio estimator involved with a more naive derivation (as discussed in the OpenMC docs), but does create issues with some fixed source problems featuring large near void regions. In pathological cases, this can result in negative fluxes and can cause instability.

It would be good to implement alternative estimator options and to allow users to specify if they want to use an alternative estimator in settings.