Belief constructors

It might be nice to have some constructors for the beliefs to answer questions like "how do I make a particle belief from a SparseCat". But, it's not 100% clear which way to do this.

## Approach 1:
```julia
ParticleCollection(d, n::Integer; rng=Base.GLOBAL_RNG) = ParticleCollection([rand(rng, d) for i in 1:n])
```
(and similar for `WeightedParticleBelief`)

## Approach 2:
```julia
function WeightedParticleBelief(d)
pairs = collect(weighted_iterator(d))
return WeightedParticleBelief([first(p) for p in pairs], [last(p) for p in pairs])
end
```

and a similar `ParticleCollection(d, n::Integer=100)` that allocates unweighted particles in proportion to their weight.

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Approach 2 is nice because it's deterministic and low variance, but it won't work for some distributions (e.g. continuous ones).