Accessing ancestor indices

[THargreaves]

The current implementations of resampling do not expose the ancestor indices to the callback function.

I can see two approaches to dealing with this:

• We introduce an "intermediate" object for the particle filter as in KalmanFilters.jl which stores the ancestor indices for each step.
• We include the ancestral indices in the state (as in the PMCMC literature)

[THargreaves]

This is now handled by the ParticleContainer.