Methods for determining state types

[THargreaves]

A few parts of the code (mainly callbacks) require knowing the type of the state before running the filter. This leads to a load of ugly, manually written code like

    particle_template = GeneralisedFilters.BatchRaoBlackwellisedParticles(
        CuArray{T}(undef, D_outer, N_particles),
        GeneralisedFilters.BatchGaussianDistribution(
            CuArray{T}(undef, D_inner, N_particles),
            CuArray{T}(undef, D_inner, D_inner, N_particles),
        ),
    )
    particle_type = typeof(particle_template)

It would be nice to have methods that you could recurse on to calculate particle_type.